One of the best gameplay engines I've had the privilage of expiriancing.

User Rating: 9.7 | Dungeon Keeper PC
It isn't graphically stunning now but the gameplay is brilliant. It was followed by a disappointing sequel which had better graphics but 'reinvented' the gameplay and ultimately ruined the franchise. I am of course talking about Dungeon Keeper. If you have not played this game then you simply have not lived. The game is set in a fantasy medieval era. Where the good peoplelive above ground, and the evil creatures live in dungeons underground. The aim of the game is to tunnel through walls using your worker imps, and build specailist rooms, attracting different types of creatures to your dungeon. You have to work hard to maintain a ballance to pay your minions and train them up to conquer rival keepers and the lord of the land. Here is a link to the Gamespot summary. Check out the demo if you haven't seen it before.