The LocationI live on a moderately populated large island on the coast of western Canada (hint, it has our provincial capital on it). While it is an island, it's not isolated or small enough to avoid infection in these days of air travel and ferries. Anyway, the ideal location requires a few things.
1. Close to, but not too close to, a major town or city.
This is to make getting supplies a little bit easier. Though dangerous, we will have to scrounge, and you can't scrounge much from a gas station in the middle of nowhere. It has to be far enough away so that we don't get overrun by infected people from the city, though.2. Near a river, and preferably the ocean as well.
The river is to run micro-hydro for power, and possibly as a source of water as well. It could be a natural barrier to attack as well.3. Plenty of open space.
First off, we need to grow food. Also, large space makes for a good killing zone. We need quite a bit for the fields, then there will be a fence and then just open space. We'll have at least one sniper/watch tower, and hopefully some (probably very crude) artillery.4. Some elevation above the surrounding ground.
A hill makes for good defense. If it can't be done, we'll make the fort extra tall and pack dirt up against the first floor.Plan A: Modified House
Not exactly the best solution, but it might have to do. If we can't get enough money, we'll need to make do with what we have.
Building the house from scratch, extra strong and with some extra security measures such as solid steel doors and heavy-duty window shutters would be better but maybe not possible.
The house will be retrofitted with wind and solar power. It probably already has a well for water. The shed contains a generator and some fuel, in addition to the extra fuel tanks.

This is the basic idea of the whole area. The outer fence is not very strong, but will already be there. The gray area is outside; hopefully just some open fields. The outer field area is where we'll grow food, though the space near the inner wall/fence will probably open.
The inner wall/fence will be built beforehand if possible, otherwise it will have to be hacked together later. Ideally we will have at least one lookout/sniper tower, not shown.
Do I even need to mention that this is not to scale?
Plan B: "Habitation"Yes, it's named after Samuel de Champlain's settlement. Get over it. This is a much more expensive, but larger and more secure fort. This time I did it in SketchUp. It looks better than the Paint drawing overall, but I'm really bad at getting things to scale and symmetrical.

So basically, it has a wall, maybe half a meter to a meter thick. Four towers- one big, one tall sniper tower, and two small. The small ones can hold two, maybe three people, the big one would be built big enough to accomodate a small cannon.
The red rectangle is where parking would have been if I made it wide enough, which I didn't. You could probably squeeze 4 or 5 vehicles in there. But if the whole thing was wider, you wouldn't have to.
Each side has a basement, probably linked with the other side somewhere. The basements are for storage, mostly. There would be solar panels on the roof, as well as wind turbines and if possible, micro-hydro. A generator would be installed, and fuel and water tanks would be buried in the middle part. Of course there would be a well, too.

As you can see, the buildings have windows, but only on the upper floors. However, it would probably be fine to have them on the bottom floor as well, since if zombies get inside the wall you're pretty much screwed anyway. You can sort of see how the roof forms a battlement to shoot from. It would probably be extended above the gates as well. The garage/workshop would have one or two large doors for moving in vehicles; the other buildings only have one steel door but would be linked with all the other buildings through doors and the large basement (which is sort of an H-shape, beneath the buildings with a narrow "connector" strip). My layout probably sucks, but oh well. It's my first try making a zombie fortress. The fort is designed to hold 50 people, max, if you literally cram them in, 25-30 comfortably.

This is the overall plan for the whole colony. If possible, other structures would be erected beforehand, in this order of priority: Outer Fence, Outer Wall, colony buildings. The fields would be used for farming before and after Z-day, the killing zone is just that. The colony would be built within the outer wall, which would be made of concrete if possible. If desired, we could add a trench between the outer fence and the fields.
Now here's where the defense gets a little different. The big tower is designed to hold a small cannon, improvised or otherwise. In a pinch a catapult or even potato cannon would probably work. A guy in the sniper tower with binoculars spots a horde of zombies approaching the outer fence. Using the intercom, he calls the artillery crew and gives them the location. They fire munitions at the zombie horde, and take a lot of them out. Of course there are flaws. Practicing is probably illegal and would at least drive the neigbours insane. Artillery is kind of hard to come by. It would destroy parts of the fence. However, snipers on the tall tower would be the only other way to engage anyone near the outer perimeter. In fact, snipers would be used against small groups that don't necessitate artillery.
Anyway, the fort would have solar, wind, and preferably micro-hydroelectric as well for power (with large batteries and a powerful inverter). Electrical tools and appliances are preferred in this case. Fuel supplies will likely be limited, but power is generated from natural sources, and we can keep doing it until we run out of spare parts (which we will stock and scavenge). We will also stock plenty of long-lasting food, water, medicine, and various supplies. Tools, parts, even raw materials like sheet metal and wood. If necessary, we could bury supply caches near the fort and dig them up later. Less-than-legal weapons may need to be obtained, especially assault rifles and heavy weapons. Nothing sends raiders packing quite like an RPG round destroying their truck!