They say people's homes are their castles -- of course, were this true, this article would probably be unnecessary. People know enough to run away from zombies, people even know enough to hide from zombies, but we still need to be thinking about more. This article explains some steps that you can take to turn your home into a castle.
Let's think of the ideal shelter. I understand that the chance of finding a hideout like the one I'm going to present is slim to nil, but I'm only presenting the most logical shelter possible if I were tasked to build one with limited supplies and money. Sorry, but that means no nuclear missile silos with bodybuilders defending it or anything like that. Note that I'm assuming we're working with a stereotypical shambler- slow-walking, no real dextrous actions possible, will infect you if it breaks the skin or swaps bodily fluids (i.e., if it brushes up against an open wound or if you have some weird fetish and attempt to French kiss a zombie).

The front door should be as thick and heavy as humanly possible to still open with reasonable speed- for obvious reasons. I’m not talking about a bank vault type of door, one that needs a small army and an industrial winch to open, just a plain steel door with some iron plating bolted to it will do quite nicely.
We should have a basement with two entries and exits, one in the main floor of the tower and the other on the exit, root-cellar style- hopefully flush, floor-mounted ordeals, the kind designed to have an inset handle and to have a rug thrown over it and forgotten until the Moors or the Huns stop by to kill you and take your livestock. I don’t know how well stealth works with zombies, seeing as how certain senses as smell seem to be much enhanced in zombies (from what we’ve seen in the media, anyways), but an entry like this, made of reinforced steel plating with a good lock over it, should repel even the most ferocious zombie assault. In the basement you should store enough food to last at least a week for each person (if applicable… of course, this is in addition to all the supplies and necessities you’ll need to last through an extended apocalypse), if you’re forced into a siege-type situation, you’ll have enough supplies to tunnel your way out/set up an ambush/hold an LAN party and with time to spare to do ALL of those. Of course, you should also stock enough weapons/flares/whatever else you may need in a pinch.
As to the main section of the tower, ideally it’ll be mostly stairs. This may sound rather silly, to be wasting space, but lots of stairs provide a place to hold your ground and it gives a ridiculous advantage to you- only one zombie can come at you at a time, and all that space gives you lots of room to set up multiple barricades, traps, and/or weapons platforms. If you lose one barricade, move up 15 feet and get ready again. That said, if the tower is especially tall, it’s alright to have multiple ‘rest stations’ or rooms along the way. Each should have a very small stock of food, water, and ammunition if applicable. A few chocolate bars and a bottle of water or pop will due. This is not a siege centre- if you’re holding them off and happen to be knackered; it beats fainting from dehydration or running up the tower to have a drink. Windows along the steps are very good, but keep about the size, a little bigger, than the size of your own head- the chance of a zombie climbing up a sheer wall is very slim, but I won’t rule anything out with the jumpy “sprinter” types possible. Don’t use glass in the windows- they’re that small only to add light and to allow you to, if necessary, fire or throw projectiles out of the tower against zombies and/or roving bands of Mad Max-esque motorcycle gangs. If you must, use heavy shutters on the windows.
Now we get to the heart of the miniature fortress, the living quarters. They will be neither comfortable nor spacious- if you want nice and big, take over a mall so you can talk to the crazy gun shop owner across the street. These plans are designed for one to three people- not for a whole troupe of people. The fortress I envision is basically impregnable to zombies, and with a realistic floor plan to boot- at the sacrifice of much personal security. Allocate no more than one-third to one-quarter of all the space to living quarters- you’ll need maneuvering space and lots of room to store food and other supplies. Living quarters includes sleeping areas, a kitchen, a lavatory and anything else you may need in your little house. What to keep in your quarters, I don’t care, but keep the basics- comfortable sleeping quarters (meaning hammocks, cots, old mattresses, whatever), cupboards for food and stuff you don’t feel like stacking in corners, a small butane stove (or electric grill, mini fridge, and espresso machine if you’re so gifted as to have either a generator and fuel or a power supply), most of your weapons and sundry supplies, and pretty much anything you want on hand on short notice. Ideally, the zombies will never reach this stage, and if they do, you’re likely already dead, so make your refuge as comfortable as you want.

Ah, now the fun bit- how will we arm ourselves and defend our fortress?
First, let’s assume that zombies are in your fortress, so let’s look at the best weapon for this situation- polearms. It’s the only sort of weapon I can advise of, at least in close quarters like these. Ranged weapons lose their effectiveness when the zombie is 5 feet away, and something like a knife or club isn’t desirable because it forces you to both get in close and fight them off, one by one. The narrow stairways fix the second one, but the first is worrying, because our only real advantage against shamblers is being able to hit them at a range comfortable to us. Yes, a shotgun works very well, but I rarely advocate firearms simply because you’re screwed when you run out of ammunition. The absolute easiest polearm you can make on short notice? Get a hollow steel pipe, and saw off the top at an angle or hammer it to a point. Instant jabbing stick. Or get a broom pole and a kitchen knife, and securely duct tape the knife to the end of the stick. Not as nice as our hollow spear rod thing, but it still slices and dices and even juices with the proper attachments. Polearms are exceedingly useful because you can hit them without having to get close, and they’re great for keeping stuff back- if you can’t hit the thing, plant your spear/s against the ground and let the damn thing impale itself. If you really want to you can carry a sidearm like a knife, but make it big- destroying the thing’s brain with a dagger is going to be impossible. A machete, though? Now that’s a knife. If you want to hit something outside of your fortress, firearms (rifles in particular, no shotguns) work great in THIS situation, simply because it’s the only easy way to effectively blow something’s head off at 150 yards. If you don’t have a rifle or firearm, I’d suggest building a mounted crossbow. Yes, you read that right.
Mount it to a plank of wood so you can drag it around, if necessary. Basically, you’re going to want to find something very stiff but springy- a thin bar of steel, or many thin sheets bolted together, works well. Affix a stout metal cord to both ends, and, well, build a crossbow. I’m no weapons expert, so I’m not going to tell you how to build it, because, hell, your guess is as good as mine. Make sure to have a channel to fire the bolts you’ll have to make or find (long concrete screws work well…), and fire it a lot after building to make sure it won’t explode when you need it most and so you can get its’ firing patterns down pat. Of course, you could avoid all of this and just find/loot/buy a hunting crossbow, but who am I to tell you what to do? It’s more fun this way.
At closer range, try heavy rocks. If they’re pounding on the door, drop a small boulder on them… simple but brutal. Don’t bother with such silly things as hot oil or burning gasoline- zombies can only be destroyed by completely removing the brain or rendering it useless. Fire doesn’t do that very well or quickly, and hot oil is useless because it burns flesh and causes pain, that’s it. Won’t damage the brain. The absolute most you could theoretically achieve with fire is to burn the limbs or atrophy the muscles, but that would take a very long time and present a major fire hazard as it stumbles around. In other words, don’t bother with fire unless your opponents happen to be human (like the aforementioned Moors, Huns, and/or Mad Max-esque biker gang).
Defenses are even more important than weapons- you don’t need weapons if they can’t get in. First, dig a ditch around the foundations of the entire fortress, or at least a large hole around the entrance- it’s brutally simple but, from what we’ve seen (what with zombies being ‘un-dextrous’ and prone to being shoved/and or pushed over), a shambler shouldn’t be able to pull itself out of a 5-foot hole. Just dig the ditch at least 4 or 5 feet deep, and wide enough so a few at least can fall in. This defense works great until the zombies start stacking up and they end up climbing over each other, defeating the purpose of the ditch. When a few zombies have fallen in, go to the roof and have a field day with your boulders. Have a plank by the front door for exiting over the ditch if you need to.
But what happens once they’re in? Although it’s pretentious to expect you to have the materials on hand to build one of these, an iron or steel portcullis that drops down in front of the stairs will keep them out indefinitely. Unless they learn how to manually raise it (a padlock will fix that), a step as simple as this could theoretically keep them out forever. Assuming you don’t have one of these (and you likely won’t), barricades are your friends. A dresser thrown down a flight of stairs will work very well for a little while- more permanent barricades (as opposed to cinder-block walls, the sort of thing that you’ll never be able to leave) take time but are worth it. For example, get a long wooden table or piece of plywood, and nail on some chocks of wood as handholds along the edges. Then cut it so it’s a little taller than the stair-to-ceiling measurement. Then cut some slots along the length of the board, roughly at eye level. Keep them wide but short, sore of like an eye-slit- if you can force your hand through it’s too big. If the zombies come up the stairs, jam this in the stairwell and use the slots to stick your spear and/or weapon through and try to keep the bastards back. Smaller versions of this can be built in the style of riot shields, to serve the same purpose as this barricade.
Offensive defense systems are great too, but they almost always need to be manned and are more difficult to set up. For example, you could line the pits you dug outside earlier with sharpened spikes, to rip the zombies a new one. Or you could build a sturdy barricade with dozens of nails sticking out of it, leaving any zombie who attempts to break it down with their hands with bleeding stumps. Hell, you could even go Home-Alone on their asses and hang paint cans on ropes on a banister to swing at the walking dead’s pretty little faces. Be creative, and utilize what you have in your environment.

Any ideas what it could be yet? I'll help you out- an old belltower, maybe 3 to 5 stories high. Doesn't have to be a belltower, it can be a lookout, a guard tower, a castle turret, a lighthouse, whatever. There are two major requirements for it- that it isn't a ramshackle steel pole or wooden arrangement (ideally it'll be built out of stone, simply for stability’s sake and for durability), and that there is a good method of getting up the tower that is convenient to use in a pinch but is easily barricadeable (a narrow spiral staircase works very well for this). This is why an old belltower fits the bill exceptionally well.

Of course, if you read all that, you’re probably chucking to yourself, thinking, “The odds of me finding a belltower with 6 month’s worth of food and weapons in it are…?”. And that’s to be expected, because you could never hope to find or build something this perfect for the situation. But use the features I mentioned to your advantage when selecting and reinforcing a shelter- seek higher ground when attacked, keep your distance from the zombies, always try to engage them one-on-one, ensure your defenses are sturdy and plentiful, and most importantly, don’t get bitten.