A week ago I played what was known as the "2D fortress" because my copy was from Oct 2007. Well after a few rounds with that (I'm not very good) I updated to the latest and I must say there are quite a few changes!
Now there is Z-depth supported so you can dig down or up in a mountain. Outside you can stack walls and make towers or even a castle. Water flows downhill and magma can bubble up if not corked properly. Something really cool is how the geological structures are now very close to real-life layers. On top is sandy loam which you can farm, then you get into the sediment layer and other rocks below. You can pick your fortress location in the world with fine detail so it's not just a sheer cliff face to the east... it can be a small hatchway coming out of the top of a plateau, or under a plain, or nearly anywhere. There are floors supported, which could be a grate (catwalk) or glass (greenhouse) or just rock/dirt.
The real fun is creating large engineering feats, such as plumbing an entire tower with drinking water, making a surrounding moat and drawbridge, creating elaborate traps for the AI enemy to fall into like a water tower above a sealed room with triggers for drowning, etc.
If you can get past the ascii graphics and the keyboard interface, this is one fun and unpredictable sandbox type of game!