Dungeon Heart

You always start out with a Dungeon Heart, as it is the source of your evil  power. If your Dungeon Heart is destroyed, you lose. Other then that, it doesn't serve any purpose.
Some spells revolve around the Dungeon Heart. The Hold Audience spell will summon all your minions to the Heart, which is helpful if you find it under attack with no monsters within easy reach. The Armageddon spell will not only summon your monsters, but all the creatures (with the exception of chickens) on the map to your Heart. Obviously, if you choose to cast Armageddon, you had better be pretty sure you've got the creatures to win the battle otherwise, you'll be in a world of hurt..

Entrance Portal:
                          Cost per tile:  N/A
                          Room Health: 4000
                          Monsters it can attract: None

 Portals are the most common - and sometimes the only - way monsters will enter your dungeon. The more portals you control, the more monsters you get.
While this is rarely an issue during single-player games, it's important when  playing other humans, who will often go straight for your portal - you should have your Imps reinforce the walls as soon as possible.
While it's tempting to capture portals as early as possible, you should first build up your dungeon so that you'll attract the most powerful creatures first. See the individual room listings for information on which monsters are attracted by which rooms.