alright, those images/videos were only for imagination trigger, now the de Jure part.
What's the Dungeon look like from the outside?Semi-natural cave entrance with stairs where necessary, gradually designed to facilitate an army in marching inward and outward.
Structure: What are the rules and floors made of?Granite, sandstone, limestone, materials with enough hardness to take long-lasting form when carved.
Lighting: What lights the Dungeon and how dark is it?Dim enough that minions can still hide, but with magical illumination where necessary using fluorescent crystals. The monsters may need a certain air conditioning to keep their hotheadedness low and their rage controlled. This doesn't sound going well with heat-producing light sources.
Atmospherics: What sounds and smells greet those who enter.An overwhelming scents from special incenses that act as the monsters' aromatherapy but might smell weird for upworlders. The temperature and humidity might change from chambers to chambers in order to facilitate each monster lord's preference but overall cool. The imp workers are chanting certain songs in unison as how Baganaria taught them during their probation employment.
Hoard: What makes up the piles of treasure in your hoard?Magic crystals that makes the magical illumination highly sustainable and can also be used as an alternative heat-source necessary for the Smithy when refined.