When a new room is built a flag appears on the first tile that was laid down. The flag has a picture icon on it telling you what type of rooms it is.
Next to the icon are three bars:
- Health (top red bar)
- Capacity (middle green bar)
- Efficiency (bottom white bar)
The Health of a Room will begin and remain full until a rival Keeper's Imps try to claim it. There is a large time before the room's health starts to decrease, this time increases the larger the room is, meaning it is easier to defend larger rooms from capture. Once the room has been drained of health, the room belongs to the enemy Keeper and its health will return to full.
The Capacity of a room depends on how many creatures or objects occupying it. If nothing is occupying the room, the bar will be empty, and will increase for each creature and object occupying it until it is full and can contain no more. If the bar is high then it is nearly full and you should consider expanding.
Examples of objects that use up capacity are:
- Gold for the Treasure Room
- Chickens for the Hatchery
- Spells and creatures for the Library
- Traps and creatures for the Workshop
- Corpses in the Graveyard
The efficiency of a room means how much it can hold for its size. Efficiency depends on the number of surrounding room tiles and walls. The least efficient design is a single tile not surrounded by walls or room tiles of the same type. The most efficient rooms are rooms filled with only one type of room tile and Fortified Walls and doors. As can be seen from the above image, the Training Room is very efficient (but not totally, as there are many doorways leading out of it), and the Hatchery is less efficient because (as seen on the minimap) it is surrounded by some floor tiles.
"Two rooms can be the same size and yet one of them produce ten time the amount of the other. Why? One word, Efficiency. Build your rooms square for maximum efficiency and then fortify the walls and put doors on the entrances, Your minions will feel safer and do more in an efficient room. Try to avoid building rooms that are one tile wide, or a strange shape. Graveyards are unaffected by efficiency and can be any shape you want."
According the the Dungeon Keeper Manual:
"You can build a room any size and shape you want, but that room might not be as efficient. Efficiency directly affects the rate at which a room works. A more efficient Hatchery, for example, produces more chickens, a more efficient Library researches faster. A totally inefficient room is a single tile on its own. It is, in fact, negatively efficient.
If a room tile has another room tile of the same type placed next to it, its efficiency automatically increases. The more room tiles of the same type surround it, the more efficient it becomes. The most efficient room tile is
one that is completely surrounded by room tiles.
In general, large, square, enclosed rooms are the most efficient. Sometimes it’s impossible to build rooms in the shape you want—the Efficiency rating helps you make the best of the area you’ve got.
Efficiency also affects the fire trail which travels around the perimeter of the room. The higher the flame, the more efficient the room."
Examples of how efficiency changes capacity:
- Treasure Rooms can store more gold (less efficient rooms can only store small bags of gold, while more efficient rooms can store larger piles of gold, and even gems and treasures, meaning more gold collected on each tile).
- Libraries will research quicker and allow for more stored spells and researchers.
- Training Rooms will allow more units to train and will train them faster. The price of training will not increase so training a unit to level 10 in an efficient training room is both faster and cheaper.