Here's a map of an abandoned underground keep that I drew to test a new sketch book I got today.
This map has a big deep chasm at the center, and there's a pool of water at the bottom of it. An underground lake or river perhaps? The stairs built into the chasm's wall lead down into the keep. The stairs are old and crackled. Anyone stepping on the stairs should be careful as the stairs might collapse under too much weight or pressure – especially if someone jumps or falls on them.
On the Northern and Eastern walls of the chasm there are arrow slits that were once used to keep watch on the keep entrance and defend against uninvited guests. Who know who, or what, might be lurking out of the slits now...