One of the most fun RTS games I think I've ever played.
Using your mouse, you control your own dungeon and the creatures inside and destroy anything and everything else in your realm. There are a large variety of creatures, traps, spells, you name it, to add into your dungeon. Using imps, dig through the ground to find gold for your treasure room and dig out rooms for your monsters to live in, train in, eat in, ect.
You won't always be the only Dungeon Keeper either, so there are times when you need to teach rival dungeon keepers where they belong. Or if the Lord of the Land and the other "heroes" get a little uppity, let your monsters take care of them. Or, drop them into your torture chamber and slap them around a bit. Don't let them into the heart of you dungeon though, that's where your heart is also!
I can easily recommend this game to anyone and everyone everywhere.