The Treasure Room adds space for storing gold. It is cheap to construct, at 50 gold per tile, and is one of the "basic 5" rooms, usually available at the beginning of any level. As the efficiency of the room increases, the more gold it can hold on one tile, ranging from a small pile of gold, to a hoard of coins, gems, ceremonial swords etc.
If the dungeon has multiple Treasure Rooms, the most recent room to have slabs constructed or sold is the "active" room— the one from which all the player's expenses are deducted. Even when a creature collects its payday from a different Treasure Room, the gold is deducted from the "active" one, for instance.
The Dungeon Keeper Goodies Disc contains wave files of Mentor dialogue that were cut from the game, some of which mention a 'Counting House' room. This is presumably this room, or an early version of it.
Notes on Efficiency Edit
A Treasure Room bounded on all sides by fortified walls and Doors and/or Guard Posts has 100 percent efficiency, and a correspondingly high gold capacity per tile. However, if it is filled-up, there is a noticeable glitch which prevents that room from being recognized as full by the Imps, and they may start running around it in confusion, wasting time.
So in the interest of keeping the Imps focused, it may not be the best idea to create a 100 percent efficient Treasure Room, and furthermore, it's a good idea to make sure that the Treasure Room most convenient to the Gold and Gem Seams is the "active" room, so that it depletes while the Imps fill it up.
| ||Barracks / Bridge / Dungeon Heart / Graveyard / Guard Post / Hatchery / Lair / Library / Portal / Prison / Scavenger Room / Temple / Torture Chamber / Training Room / Treasure Room / Workshop|
| ||Casino / Combat Pit / Dungeon Heart / Graveyard / Guard Room / Hatchery / Hero Gate / Lair / Library / Mercenary Portal / Portal / Prison / Stone Bridge / Torture Chamber / Training Room / Treasury / Unholy Temple / Wooden Bridge / Workshop|
| ||Dragon Chamber / Infirmary / Kitchen / Crypt / Hive / Tomb / Summoning Chamber / Parlour|