The Penthouse is a structure at the lowest level of Welltower, even underneath the bottom of the Shaft. It occupies levels 880-883. It is richly furnished and has plenty of food. It has good lights and ordinary beds, sinks, toilets, and showers. Because it is so deep, it is very hot and humid. It has a special new entrance to one of the express ramps which Jolley built. Chell, Mister Jolley, Welswept, and others of the elite live in the Penthouse along with their guards and staff.
The Penthouse seems to have a germophobic no-touch culture.
The Penthouse is completely flooded when the aquifer ruptures.
Kaehl looks for the Penthouse because he believes it can cure Selda of her ailment. When he finds it he is given food but thrown back out without Selda. Kaehl later returns on orders from Herstand and has to flee from it with Jolley when it starts to flood.
Level 880 contains the residences of Tower administrators. 881 contains supplies, 882 offices, and 883 the homes of the most important Tower leaders.