A complex and addictive game, but very hard to learn. Oh, and it kills your computer's performance.
It's a massive "sandbox" strategy game. First you create the map, which can take a long time on slower computers. Then the action begins. You get seven dwarves and some items, which you can choose yourself. There are a lot of things to do, and by "a lot" I mean so much that it takes forever to learn all of them.
You will probably start by chopping some wood and digging the first few rooms of your fortress-in-the-making. Then you can build workshops of which there are over 20, which you can use to make things like furniture or weapons. You can also build things like bedrooms and dining rooms for your dwarves to use. The happier they are, the more work they will do.
There are about 70 different skills for your dwarves to learn, such as mining, carpentry, hunting, milling, health care, animal training, wrestling and so on. Dwarves gain experience every time they work, and a high skill level allows them to make products faster and with a better quality.
After a while, you start to get caravans in your fortress. You can trade with other civilizations to get useful items. You will also begin to get immigrants at some point.
Building a fortress isn't entirely safe, though. Enemies can attack you at any time. You need good military and traps to get rid of them. Your dwarves can also get killed by magma, which you can find while digging. Magma can be used to power some workshops, but if you're not careful, your awesome fortress can be burned to ash in a few minutes.
You will also find another game mode, the Adventure mode. Its very similar to old roguelike games and should be very fun for fans of, for example, NetHack. I haven't tried it myself, so it's don't included in this review.
The game also has its problems. It uses colored ASCII graphics, which some players may not like - there are graphical tilesets available, however. It also has the highest learning curve found in any game, as it's extremely complex and has no proper manual. A tutorial is basically a must to learn even the basics.
The other problem is that even though it only has text and symbols, it's very CPU-heavy and requires a powerful computer, unless you don't mind the slow speed. It's not unplayable at lower speeds, but may get annoying after a while. Even if you have the best computer parts available, it will slow down as your fortress grows larger.
If you have enough time and patience to learn how to play, you will find a very addictive and complex game. Even if you have a huge fortress with hundreds of dwarves, you can always find something new to try, such as building a 20 floor high tower from glass.