Here your monsters can improve their skill rating without the dangers of actual battle. The dungeon heart is the single most important room in the game. If this is destroyed, by good or by evil, your game is over. As with any good RTS, everything you build costs resources. In Dungeon Keeper there are only two resources that you have to keep your eye on, gold and mana.
Gold has to be mined from special ore squares on the map.
After your imps have mined an ore vein, they'll gather up the gold and take it to the nearest treasury. If you don't have a treasure, then you won't be able to get any credit for the gold that you've mined, so it's really important to take notice when your existing treasuries are getting full. Every now and then you'll be fortunate enough to find an gem square. These areas can be mined continually for gold and never disappear. If you find one, make damn sure that you protect it from anyone else who might be hanging about.