Earlier this week Jeff's Gameblog posted up some thoughts on the 2D-ness of many dungeons, along with some really great examples of maps that bucked the trend. I had mixed reactions to the article. Jeff certainly has a point when he says "Even many incredibly awesome dungeons suffer from a basic problem in that we, the dungeon designers, allow the graph paper to do some of the thinking for us."
But wait a minute.
Take a look at the world around us and the spaces we live and work in every day. Most of them are... flat. Sure there are exceptions, but humans (and in the fantasy world humanoids) are generally upright ground-pounders. There's a good reason things are flat. Flat is usable and efficient! Even a lowly kobold would rather use a corridor instead of stairs if he could. And who wants to carve all those steps when a straight passage will do? We're all inherently lazy creatures. Be honest now, would you rather have the office water cooler just down the hall or up two floors with no elevators?
Nooooo! Flat is boring and lame! I can hear the protests now.
I agree. My point is forcing vertical features just to have an interesting map or present a movement challenge isn't enough. One should also think about why the vertical feature is there and what impact it has on the dungeon as a whole. The kobolds aren't going to make use of a big vertical cave, except perhaps as a dumping ground. A giant spider, on the other hand, could make such a cave a perfect home. Here are a few ideas for non-flat spaces and how they might be used in the dungeon environment.
Connections - Stairs, ramps, lifts, and ladders. Someone took the time to build or carve a passage between two elevations. The connection should reflect the builder's size and movement characteristics. Stairs or ladders created by kobolds would be uncomfortable for humans, while stairs carved by giants might be an obstacle. Anyone that goes to the trouble of creating connections is probably going to use them with some frequency. Wandering monster checks are a must. Lastly connections serve as good defensive barricades. A well designed stair can give defenders a positional advantage, and a well placed pot of boiling oil can ruin any group's day in the confined space of a spiral staircase.
Defenses - Even underground a well-placed wall is a significant barrier. Fortifications that work outdoors work just as well, perhaps better, indoors. Imagine the pictured tower or a moat-protected, curtain wall, complete with drawbridge in the middle of a huge cavern. The defenders can use their elevated position on the wall to rain death on any attacker, while the waters of the moat hold who knows what horrors? Even a basic pit trap can be something more than a simple hole in the ground. Who's to say it doesn't lead to a chamber in a lower level?
Grand Displays - The big temple, the theater, the throne room. Places like this naturally lend themselves to galleries, balconies and other vertical spaces. They are often a focus of activity in an underground community, and as such are usually occupied and/or guarded. Access should suit the purposes of the space, and an area like this could serve as a hub, connecting several different areas of the community.
Natural Caves - If you want random vertical spaces, natural caves are your friend. Just take a look at these maps of Crystal Cave and Soldier's Cave in Sequoia and King's Canyon National Park. They provide a glimpse of how 3D caves can be. Natural caves are chaotic, difficult to traverse places. Fallen rock, stalagmites and narrow passages are enough to drive a cautious party insane. Many caves are created by water flow, and we all know water flows downhill. Natural caves should reflect this, featuring sloping floors and vertical drops along with underground streams, waterfalls, and pools. Passages shouldn't stop at the water's surface either. All that water has to go somewhere... Underground dwellers will be quick to make use of natural caves, saving themselves the work of laboriously digging tunnels and passages. Of course finding all the entrances and exits to such a labyrinth is difficult, so cautious inhabitants might simply wall off whole sections of a natural cave system, creating natural divisions in what would normally be a vast connected network.
In conclusion, use vertical space, but use it wisely. There are plenty of places vertical space can be used to challenge players without breaking their suspension of disbelief. On the other hand, don't take logic too far, after all (to play back to Jeff's original Star Trek theme), as Spock said: "Logic is a little tweeting bird, chirping in a meadow. Logic is a wreath of pretty flowers which smell bad."