The monsters in the dungeon are ephemeral. It's the geography that's going to stick around. Minimalist keys are fine, but use them to make memorable locales.When it comes to any adventure area, varied architectural regions and features really make discovery and investigation mean something. The physical environment, along with the things and creatures found in the dungeon, all provide a clue to its former uses and present story. Finding a new area of the level with its own feel and mysteries gives some of the thrill that, in a more standard design, can only be found by going down to a new level.
A while back I presented a table of six questions that could help define an adventuring area's past and present. Now here are some ideas for designing underground areas in ways that give the players an idea of this history.
Width: Are the areas extra-wide? (Built to handle impressive processions and ceremonies, large bodies of troops or workers moving in multiple lanes) Or are they very narrow? (Built to economy; natural passages; intentionally restricting intruders to a dangerous single file.)
Design: Are rooms and passages laid out at regular angles? (Indicates the builders valued planning and a sense of order.) Or do they veer and jut at crazy angles from each other? (Indicates haphazard construction: either slowly over time, rapidly without organization, building upon natural features, or intentionally as a way to confuse intruders or honor a Chaotic worldview.) Consistent features in an area's mapping, like rounded corners, oblique or right-angled room layout, complex or simple shapes, single or multiple floor levels: all can make an area of the underground distinctive.
Features: Are there recent tracks, layers of dust or grime, blackened marks from a fire, water marks, standing water, areas worn smooth by centuries of passage? Have rails, pipes, or gutters been laid to convey air, water, and useful goods? Is it wet or dry down here, warm or chilly? Does mold and fungus grow? Are there tools, furnishings, machines, doors - and what themes does their construction follow, and what condition are they in?
Explorers of my Cellars of the Castle Ruins - my home-brewed first level for Castle of the Mad Archmage - have found the standard granite faced and dressed dungeon construction; a hollowed out area filled with floor-to ceiling walled storerooms, made of red brick and half-timber and having a cut-cornered design; another area of yellow brick with strange floor decorations; and a crudely carved area of former wine cellars. And there are still several special areas as yet undiscovered by anyone ...