Restructure Gehennom as follows: the main trunk consists of the Valley, then 5-7 filler cavern/lava levels interspersed with two bribable demon lairs, then the Vibrating Square level (also cavern/lava), then the Sanctum. The Vibrating Square level contains six revealed portals, one of which leads to the Wizard’s Tower, one of which leads to Vlad’s Tower, and the others all lead to demon lairs in which you can’t leave the level until the demon is defeated.