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.