Once you have the first five rooms built and a comfortable amount of gold, start having a few of your Imps reinforce the walls (you really need a solid number of Imps at the beginning of the game - eight to ten is a good start). Strong walls are the key to a solid dungeon: Your enemy can't get through them without casting Destroy Walls.

As you expand, make sure you cast Sight of Evil to see the area your Imps are digging out. This is to avoid unwarily stumbling upon caverns, water, or lava - which can't be protected. If open areas can't be avoided, make sure to build rooms on the border where your minions spend the majority of their time, such as lairs or training rooms. And if you're building multiple rooms in wide-open areas without the benefit of walls, don't place differing room tiles next to one another; leave a buffer of at least one square to ensure maximum efficiency in both rooms.

Building from here on out should take advantage of what you've already accomplished. For instance, if you've built the larger rooms to extend to the edge of the map, you can build smaller rooms between them that will be inaccessible to enemy explorers. The reinforced walls of the border rooms will serve as blockades to the smaller rooms.

The value of reinforced walls can't be stressed enough. Not only do they protect your dungeon, they make your rooms more efficient. On a related note, impenetrable rock, while it does protect your rooms, does not add to their efficiency. To keep your rooms at their maximum level of efficiency, keep a small buffer of wall between your rooms and the rock.

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