Dungeon Keeper 2

The Good
There is simply nothing out there like this game. Well, that's not true -- Black & White and Startopia come close -- but DK and DK2 were first. It's a RTS, it's Sim Dungeon, and more! And the evil narrator who advises you as the game goes on is wonderful, dryly funny when you're doing well to downright threatening when your game is going poorly...

The Bad
It's got a sharp learning curve, and demands careful perusal of the manual. Even then, you can have fun in the "My Pet Dungeon" mode, that lets you tunnel around and learn as you do.

The Bottom Line
The game is basically a real-time-strategy game, viewed isometrically, with some interesting twists thrown in by the ability of you, the Dungeon Keeper, to add traps to the tunnels you dig. You must recruit creatures through Portals you capture, and mine gold to pay them, and build rooms to house, feed, and entertain them while you get them to achieve your nefarious goals.