Before we get into details, lets go over some elements that need to be taken into consideration.

Location: What is the climate like? Is it close enough to any possible raiding locations?

Accessibility: How hard is it for you and party members to get in? Do you require any special vehicles or gear to reach the spot? You'll often find the least accessible spot is the most defensible. Try to keep a practical balance.

Defensibility: How hard is it to keep unwanted things out? Can you attack the enemy without them being able to counterattack? Does your location put them at a disadvantage? Are there any small holes where somebody could slip in? Can you funnel the enemies into one entrance so you have less areas to defend? Does the area have many windows where you would need extra barricades?

Visibility: Can you keep an eye on the surrounding perimeter? Will you have any advanced warning of any enemies? Does the area have an existing lookout you can use, such as a tower or rooftop?

Escapability:If your location gets totally surrounded by the undead you will eventually need something you don't have. Will you be able to blast or sneak your way out? Can you lure them away from your position long enough to escape? Do you have a giant catapult to launch yourself out?

Comfort: Does it have enough rooms for everyone? Can you heat/cool the area as needed? Does it already have supplies?

Fortification: Will it require a lot of work to make defensible?

Waste Disposal: Is there a way to dispose of feces and garbage?



Fortification:


Lines of Defense(In order of nearest to furthest):
Your outpost walls - Hopefully the undead never reach this point.
Mechanical impalement/impact device - Conserve your bullets.
Trench/Killpit - A perimeter, and/or pit near the entrance with a trapdoor over it.

Razor wire - Slows them down, trips them. Most zombies can't use wire cutters either. Easy to set up.
Landmines - KaBOOM! Be careful not to splatter yourself with undead blood and organs.
Punji pits/Tiger traps - A deep pit, usually with spikes at the bottom.
Motion Detectors - For those sneaky creatures.

Outpost sniper - Don't let them get too close if you can help it.



Defensive solution in a can: Common locations that are ready to rock and roll.

Prisons - Comes standard with sniper towers, a fence, and home furnishings. Many rural prisons exist as well.

Costco - Few entrances. Plenty of things to block them off. Plenty of supplies. Low escapability.

Airports - High visibility. Comes standard with a landing strip and non-perishable fuel reserves. Defensibility may vary.

Mineshafts - Pre-made, stable temperature, hard to find, easy to block off. Rural location.

Shipping containers - Easy set up, hard to push through, large enough to drive a truck through to use as a gate. Can be stacked as high as 7 tall(roughly 60 feet/20 meters). Metal construction means you can weld things to them.

Geothermal Power Plant - Great rural(But not too rural) location. Easy accessibility by road. Good defensibility with few entrances and service ladders to rooftops. The added advantage of electricity.

Hospital - High defensibility once entrances are fortified. All the medical supplies you will ever need. Urban hospitals will have great visibility and a heli-pad. But be warned, when the outbreak happens, the hospital is going to be the first place the freshly-infected are going to be.

Military Base - All the supplies you will need. High-accessibility through roads and airstrips. Be warned, the military may have orders to shoot on site at any unidentified subjects.


More examples of Defensible Locations.