Medium Bases

Advantages
Balconies: If you live anywhere on the second floor or higher, your balcony grants you a vantage point to spot zombies and other survivors, a sniping position (depending on where the balcony is located and the height/number of building around) and even a secure garden area. Additionally, depending on your balcony structure (and items you can salvage) it may be possible with some work to climb to adjacent units, allowing you to scavenge and expand your garden (although the possibility of zombies in those units is a risk). Also, in the event of infection high balconies are an effective method of suicide.
If you're brave enough, and can get enough soil (from scouting or other residents' flower pots) and can clear the top of the building, most apartments have a sizeable roof that allows great gardening and lots of fresh rain and/or snow for water.

Lets face it, when the sewers go you're going to have to find somewhere for your sewage. You won't want to be going in your apartment. A balcony allows you to keep it farther away from you, and is a lot safer than trying to sneak out of a bungalow, or bunker for that matter.

Other Survivors: You may or may not want to gather other survivors in these terrible times. Some survivors could be more dangerous than zombies. However, in an apartment, you are more likely to find survivors you know. They're not strangers. If you're the loner, or paranoid type, you should be able to barricade your apartment or floor, and keep yourself relatively isolated. Even your balcony area would have to be infiltrated with ropes, ladders or piles of debris, which may be hard to set up with zombies milling below.


Climate: When the electricity goes out, things are going to get cold in many countries. Due to an apartment building's size and many small compartments, the interior of the building may be warmer than smaller, less insulated buildings. However, at the same time, the construction of many apartment buildings does not allow one to build chimneys or have areas suitable to burning wood and debris for warmth.


Transportation: Many apartment buildings have underground parking. While they're often big places (and therefore could house many zombies),they are likely segregated from the street by some form of barricade. Depending on the barricade, you could clear the underground, and have access to a large stash of vehicles and their contents. These vehicles could be used for scouting into the surrounding area, or obviously as a bug-out.

Disadvantages
Rot: Depending on the time of the outbreak, your apartment could be relatively empty or jam packed. Further, and depending on the form of zombie and how infectious the strain is, you may have a lot of rotting corpses which are, essentially trapped in the building. Sewage leaks as the building deteriorates without electricity could potentially cause flooding in some apartments, further adding to the decay and potential for disease.

Outside population: While an apartment is great defensive postition (if you can secure it and have enough people), apartments are in more populated areas. That means there will be a lot of zombies outside, besieging you.

Narrow Hallways: In the case the building is overrun by undead, the narrow hallways don't offer much room to maneuver, and the rooms have one entrance/exit making it easy to be overrun and difficult to escape.

Description
During the Cold War, numerous bunkers were built throughout the United States to house and secure VIPs, government officials, and military harware and personel. They were built with strong metal and concrete walls and sometimes impenetrable doors, so keep out radiation from fallout and any other kind of chemical or biological hazard in the air. Depending on the scale and type of bunker you are in, there can be dormitories, kitchens, cafeterias, medical facilities, and even armories. Several underground bunkers in the United States capital contain equipment that send short wave transmission to possibly call for help or to warn the world of any possible threats.

Unfortunately, by their very nature bunkers are very hard to get into without codes, keys, or proper authorization. While in some older bunkers, the doors may be permanently locked or if someone is already occupying the bunker it could be virtually impossible to gain entrance without the permission of the inhabitants.

Holding off at a bunker gives a high survivability to your group.

Variants
Storm Shelters
As the name implies, Storm Shelters were built to withstand powerful storms and to keep the inhabitants safe. However these are not always able to survive an everlasting barrage of the undead.

Homemade Bunker
VIPs and doomsday fanatics alike may have made simple to extravagant bunkers on their property "in case of emergency".

Fallout Shelter
These Cold War bunkers were built mostly in the 1960's and designed to limit their civilian inhabitants' exposure to the gamma radiation and shockwave of a nuclear weapon. Relatively common, they might serve well in a zombie situation. However, many have not been maintained in the forty years since they were built and were never designed for long term occupation, leaving the occupants to chose between starvation and zombies.

Government Bunker
Housed in some of the most defended areas on Earth, Government bunkers were designed to protect Government officials from everything (EVERYTHING!). However, unless you are a government official, you are not 'elite' enough to gain entrance.

Advantages
Remoteness. Campsites are usually far away from major population centers, reducing the likelihood of the undead and bandits finding your hideout.

Easy Scavenging. The remoteness also helps with scavenging, as you are in less danger from humans and undead. In forested areas, hunting will be significantly easier and gathering food is also not a problem. Most campsites are nearby a river so fishing and water will probably not be a problems.
Natural Materials. Forests have natural materials there for you to build with. Trees can make great fortifications, and can help build other useful structures. Also, trees can be climbed to provide a good lookout. Forests are littered with hiding places too, so you would be able to hide from other humans (Hiding against zombies is not recommended.). Remember that enemies can also use the hiding places so be sure to scout around carefully.

A relatively easy location to clear out if overrun when you arrive

Many campsites allow trailers and RVs, which are generally well supplied.

Disadvantages
Remoteness. While being remote is useful, being away from society may also be a problem. You are less likely to be found by any sort of rescue, and scavenging for most manmade supplies will be harder. Also, communications from most devices (Satellite devices may still work okay) will be less likely to transmit and receive signals from a long range. Dragging any supplies (Especially heavy and necessary ones) will be tough away from society (without vehicles or carts).

Reliance. By being near a campsite, you will only be able to rely on fishing, hunting and gathering as a food source, so you have no guarantee that you will be able to get food. Also, not all campsites are near water, so it is necessary to find that as well. Farming in forested areas is difficult for many plants but it is still possible.

Possibility of already being overrun or occupied by hostile humans.

How to prepare
Though campsites are set up to give you an early start for the most basic necessities, it is not as easy to gain other useful items. These tips will help your campsite to be more successful.

Stock up on needed supplies (Especially ones not easily available there), and make sure to have them set up there before it happens. This a golden rule in preparation for using a site. Remember that supplies that you can find there are still useful to have before as you will not have to search as much and every little extra counts.

Improve your fortifications. Wildlife, and the few humans and zeds in the area can still pose a problem. Try to fortify them if they are there and add other defenses like lookout posts (In the trees) and more walls.

Make sure to have transport. Even though your remoteness is super useful and you can survive based on just what is there, there will be times when the need to go to a developed area is necessary. Make sure to have some transport (One that is good for carrying multiple people and heavy loads, preferably) to get supplies back to the camp. You will need everything you get.

Don't get cocky. The low chance of undead and enemy humans being there IS NOT ZERO. It will be low (Usually, but it is varied in areas) so it is not the biggest concern, but do not think you are impossible to find there. A fort built up to be like one meant to keep out hundreds will still be useful in the long run.

The eerie thing about every End-Of-The-World story in nearly every religion throughout history: they all mention the dead rising from the grave and walking upon the earth once more (granted, it isn't explicitly implied they're the flesh-eater variety of resurected corpses, but, better safe than sorry).
Churches, especially the older stone cathedrals built by the Roman Catholic and Eastern Orthodox Churches at first seem like ideal places to fortify yourself when the dead rise. Stone and concrete walls, high forcing-you-to-look-toward-heaven windows, thick, heavy doors, well-supplied housing facilities for the parish and those seeking sanctuary, and often with one or more bell towers that could function as a prime sniping locations. For example, many Catholic churches, not even the larger cathedrals or chapels, in the United States were built in a time when they often had to defend against human-wave attacks, in this case usually being mobs of native-born Protestants rather than zombies, and were accordingly fortified.
However, churches rank low on the zombie survival meter, just as hospitals and police stations do. This is not because Satan's soldiers are trying to destroy the house of God: it's where the food is. As the rest of society falls around them, droves of people are likely to swarm to their nearest place of worship to pray for healing or miracles - only to be followed by the shambling undead, in slow and steady pursuit of their prey.

Even if a group does manage to make an adequate defense, there are going to be internal dangers. While not all religious people are extremists, one is more likely to find extremist personalities amongst survivors making camp of a religious center. In every religion there are people who take their holy scripture as the word, and intolerant, even hostile or violent to anyone or anything that deviates from that belief even slightly. Extremist personalities are more likely to ignore logic or reasoning, which can prove a fatal flaw. As the situation begins to seem more hopeless, more and more survivors may to listen to the almost insane preachings of these radicals.

City halls are often a primary destination for a scared populace seeking leadership and shelter; as such they should be avoided whenever possible. Other areas of concern are the lack of food or weapon supplies and the tendency for modern government buildings to have large glass windows and doors. When it is impractical or impossible to avoid seeking shelter in City Hall, securing the building and stockpiling supplies are of the utmost importance. Also, assigning guards to more exposed areas is a good idea as they will be able to raise the alarm should the zombies break into the building.

Massive, football field-sized floating islands chocked full of supplies, with few if any ways for the Undead to enter, a Container Ship would seem to be perfect. With thick steel doors, medical and food supplies plus the actual cargo, these massive ships would seem to be the perfect hideout- far from refugees and infected, self-sustaining for months, and massive. They have many of the same problems as Oil Rigs, but are even more at the mercy of the larger waves.
Large container ships or aircraft carriers can stay at dock and just have all entrances sealed off. They are strong, less likely to get hit by waves, and retain safety without having to be stuck in the middle of the ocean.
The most important factor, like any Base, is having the right amount of survivors with you. That can maintain the ship and keep it well defended.

Evacuation
Large ships can have many lifeboats, just in case the ship becomes overrun and a quick getaway is needed. Aside from that, aircraft carriers are designed, as the name suggests, to carry aircraft, which can function as transport.

Hotels have many rooms that can used as a make shift base for survivors. If it is a luxurious hotel it could even have a helicopter pad so that you can radio in a rescue if available. Snipers can set up positions on various floors to scan the streets below, creating a much more safe place to hold off. Hotels have large kitchens built to serve hundreds of people and can even have a emergency generator that can be reactivated with diesel fuel.
After months or even years a hotel can become the center of a new survivor refugee camp.

Securing
Use snipers on higher levels.
Board up first floor doors and windows.
Block all stairwells with debris.

Be sure to choose the hotel carefully, as some hotels WILL be full of people who have been infected, due to the rush of people trying to get to the easiest way out of the the city/town. Thinking a helicopter landing pad is a surefire way to escape, hundreds of people will try to force their way into the building, and while a few people will have gotten away, there could easily be a few hundred zombies just milling around. It is also highly unlikely you are the first person to notice this, so be on guard for bandits or the few survivors who survived the slaughter.

The safeguard for the rich and famous a Mansion. Mansions are usually built with very elaborate and strong materials. Sometimes even built with strong brick walls surrounding the perimeter to keep out paparazzi or some lunatic. These buildings can become a very easily defended area. Along with that Mansions are built with many rooms, and can support a large survivor group. Also if a Mansion has enough land around it, that land can be converted into farmland and can support the survivors living there.
Mansions have several stories that can make it easily defendable with snipers and other long range weaponry.

Missile Silos are large underground military structures built in the event of war during the Cold War era. Many are still in commission while others have been decommissioned and had their main weapon removed. They are built to withstand Nuclear fallout and are heavily secured to prevent unauthorized access. They often have advanced communication systems that allow for them to receive orders from around the country. They may also be stocked with food, supplies and ammunition.

Office blocks are typically poorly defensible. Doorways are made of cheap, weak materials like glass, and existing security is usually designed for summoning police or scaring off burglars. Offices typically have little in the way of food resources. Several of the floors may be entirely open. Many offices typically have flat roofs with access limited to a few key stairways much more suited to the constant defense of zombie warfare. Life on the roof is also more visable to passing helicopters.
If the outbreak occurred during work hours, the offices will be overrun.

If you are looking for a zombie-proof fortress, few places can compare to an offshore oil rig. Provided you have the means to locate and reach one, you'll likely find them abandoned, since during the early stages of the crisis the crews will probably have left to ensure the safety of their families. You can greatly reduce power consumption and the chance of a fire or explosion so long as you turn off the drilling and refining equipment. The remaining fuel and natural gas should provide more than enough electricity for decades. Rigs are designed to hold crews for months or even years at a time, so there will be plenty of food, medical supplies and full amenities to recreate the comforts of home. Rigs also have desalinizers and purifiers so that fresh clean water will not be a problem for a long time. Being so remote means that you won't have to deal with swarms of desperate, possibly infected refugees. And being on the ocean means you can supplement your food rations with seafood.
There are numerous problems, however, and the aforementioned fires are only the tip of the iceberg (icebergs may also be one of the problems, especially in polar regions). Pirates are yet another problem, so remember to keep weapons and plenty of ammunition, and if you brought large, impractical-against-zombie weapons like a minigun, it's best to leave it out where potential thieves can see it and rethink their options. Corrosion of your equipment is another major worry, and as little else rusts metal faster than a sea breeze. Mother Nature will be your biggest enemy, from stormy seas, to brutal icey winters intensifed by the ocean air (where applicable).
The Offshore Oil Rig is a unique holdout point, though rarely is it accessible enough to be an option. It can be a terrific short term bastion, but for long term survival, supplies will need to come from the mainland. Maintenance of both the facility and the sea vessels may be difficult, but crucial for survival.

Tips For Survival
If you can, try to seek refuge inside a commercial or cargo plane, and make sure that the cockpit is secure, with at least a locked steel door, as it is the most secure place in the plane, but without as many escape routes compared to the cabin, with the wing escape routes. A crashed plane should not be considered, as the structure is unsafe, and would be easier to access for zombies or Bandits.

Advantages and Disadvantages
The advantages of a commercial plane, is that it is very easy to secure and, in the case of a breach, easy to clear. Remove most of the seats, so your safe room has more room for supplies. Some disadvantages are that in a breach, it is difficult to escape. There also is very little room inside planes. The advantages of a cargo plane are almost the same as a commercial plane, but there is more room for supplies.

As with City Halls and Hospitals, police stations are often a poor choice of base due to their being perceived as "safe places" by the general populace. While true in most cases, zombie attacks can send panic-stricken family members running to the authorities, including those already infected and the undead who follow.
The advantage of being heavily armed, while appealing, can quickly be rendered a moot point when the officers are overwhelmed by hungry ghouls and are unable to fend them off properly. Additionally, many modern police stations are little more than armed office buildings, and present the same problems associated with city halls being large glass windows and doors. However, certain stations are built rather like millitary bases, with stone walls, broken glass on the top of any surface to stop people from climbing in, and roadblocks stopping anyone from entering or leaving by sneaky roads. An example of these are like those in Egypt.

If it is not possible to flee a police station, stockpiling of food supplies is critical; there may only be a few days' worth in the entire building, particularly if your group is large. If the building is large, attempt to secure a single floor or wing rather than try to retake the entire facility. Arm only those people in your group able to safely and responsibly handle a loaded weapon, as the temptation to hand out guns to everyone, including children and the infirm, is ever present when surrounded by weapons. In the United Kingdom a Police Station will have a very small stock, if any at all, of firearms. A telescopic baton is standard issue for British police officers along with handcuffs and incapitation spray. On the whole, those won't be very useful.

The average stadium can hold thousands of people for special events and possibly these structures can be used as place for survivors to hold out at. Stadiums have several entrances but are built very strong, and can funnel zombies through hallways and stairs. The size of the building allows construction of an inner sanctuary, in case the outer perimeter is breached. The outer perimeter should be heavily secured and able to withstand the force of hundreds of zombies pounding on it.
Although Stadiums are huge enough to support a small city, they are not stocked with a large amount of food. If the stadium has a grass pitch, survivors could plant crops in the soil if they have seeds. These stadium regularly have no roof, crop growth could be maintained.

Often a highly-sought after base by amateurs, most modern supermarkets pose a host of problems for would be survivalists. The large glass windows and doors provide next to no protection from the undead, and the sheer size and complexity of large supermarkets can make them extremely difficult to secure and guard. Food supplies would be quickly looted in the early stages of the apocalypse. If at all possible, avoid supermarkets. If a group decides this approach, care must be taken to secure front and back entrances, as well as obtaining weapons for defense. However, a number of supermarkets in southern states seem better suited for fortification, due to smaller, higher (and sometimes hurricane-proofed) windows designed to limit the amount of sunlight streaming in, and only one or two automatic glass doors in easily baricaded places.

A warehouse is a large building designed to store large quantities of goods.

They can contain goods in copious supply. If the warehouse belonged to a grocer's or supercenter, food would be in bulk and likely a long shelf life.
They contain elevated shelves, which can provide a secure place for sleeping and recreation.
The building will have a large, flat roof which can be used for crops or a distress message. Caution should be taken not to overload the structure, however.
Warehouses may have existing fortification in the form of barbed-wire topped chain-link and heavy padlocks. The fenced in yard will quickly prove an asset in terms of recreation, food production, and defense.
Many shipping distribution centers have provisioning for fueling long distance trucks.

Warehouses will be targets for anybody who lives nearby, thanks to the possibility of easily supplies with the low chance of zombies.