general lair design for me:
mostly underground. surface structure looks like a somewhat weathered old tower in the middle of nowhere.
the underground sections are partially natural caves, but mostly carved out by use of magic. stylistically, sort of a mix of your standard "dungeon" with more modern aspects - wires and fluorescent lights along the walls, some parts that aren't seen very often by things that aren't just constructs looking like full on concrete-walled service tunnels etc
the deeper you go the less it looks like repurposed ruins and the more it looks like some sort of industrial setting. the deepest parts of it almost look like a mimicry of a town on the surface - facades that look like buildings along the walls leading to homes for the place's inhabitants, the entrance to the main room where most of the hoard is stored looking like a town hall etc
