A notice of warning, well, a few actually I think.
First of all, once you understand the game, its ridiculously addictive. But a bit 'easy' on the defense side, pretty much anything the game throws at you on an ordinary map can be shot to death in a matter of seconds if you have a few well trained crossbowmen (in effectiveness they come closer to a machinegun). The good spots where you'll normally want to embark to will become somewhat easy at that point, unless you come up with superstructures like a tower made of soap.
So if you want to keep the game to be a challenge, start creating your own goals, and I'd advise against using traps, and at a later stage crossbowmen as well. It'll keep the danger alive for a bit longer.
The other warning is a system-requirements thing. The game seems like an old DOS type, requiring nothing at all from your PC. But you might have already noticed that it can easily swallow up a few hundred mb of RAM space. Furthermore, the game needs a really, really good CPU. As it doesn't support multithreading yet you'll be better off using an older single CPU PC running at a higher Ghz than a newer 4-core system running at lower speeds. You won't notice anything in the beginning, but the more dwarves you get, the slower the game will run; the more pets run around your fort, the slower it'll run. Pathfinding is the biggest drain on how much FPS you'll get.
There are a few things you can do to keep your fort running smoothly long enough. One is a necessity anyway for everyone. You'll have noticed that cats adopt dwarves (yes, not the other way around), once a dwarf has been entranced by a cat, you'll no longer be able to kill the cat very easily, probably involving the death of the dwarf as well. That means cats will breed, and they can do so quite fast, until your fort overflows with the evil bastards. You can put them in cages though, that'd help, but is a bit of a cumbersome activity after a while. So kill off or cage redundant cats. You can always make kitten biscuits. (seriously)
Another important thing, not just for performance CPU-wise but also for the fort in general, is to come up with the most efficient building-plan for your fort. Remember, going a story up or down is just one move, like an ordinary north/east/south/west. This means that it's most probably far more efficient to use multiple floors for your operations compared to just one. Dining room close to booze and food, booze close to the still, still close to the storage for barrels and food, barrel storage close to the wood, wood and barrel storage close to the carpenter, and so forth. Create a main traffic line, of about 3-wide, cause dwarves slow down when they have to cross each other, and that's taken into account in pathfinding.
As matter of last resort, or just for playing preference, you might want to set a population limit in the .ini file so that, after a certain amount of dwarves, you won't get anymore immigrants. Around 100-150 dwarves can be really draining on your CPU. And I've found that after around 100 dwarves the work of dwarf-management becomes cumbersome and you'll dread the next wave of dwarves coming. That's often the point where I set up around 40-50 of them as masons and start building something ridiculously huge. Also, don't mint coins. Your dwarves will endlessly run around with little piles of coin to deposit in their room or where-ever. Coins aren't needed anyway.
Depending on your CPU you might get an improvement in FPS from using an improved OpenGL coding made by some fans over on their forums:
http://www.bay12games.com/forum/index.php?topic=40349.0