There are four "layers" of sublevels to the Cellar, but that definition is creating a Euclidean physicality out of a decidedly non-Euclidean space. The sublevels are all demiplanes in their own right, linked to each other, and layer hubs, which also serve as nexus points for Wardens, Gaolers, and the instance of BahbOS resident within a given network of cells.
To mitigate entry and egress, the sublevels use a wide variety of magic and artifice, and below the second sublevel, technologics is most frequently what comes into play. Many of the "cells" are, in fact, extraplanar temporal "bubbles" encapsulating an artifact, villian, Cataclysm, Apocalypse, or error made by Divine/powerful entities which might otherwise rend the strained fabric of Toworia.
It recently came to light that while the divinities of Toworia are not generally in contact with the shards of their consciousness within an Apocalypse or Cataclysm, if a faithful Scion enters one of these spaces, and draws forth a focus artifact of divine power, or frees an avatar of the diety directly, that shard can bypass the prohibition on divine entities enacted by the Tower.
Sublevel 1
The Cells
Apocalypses
Artifice
Cataclysms
Inmates
Sublevel 2
The Gemstone Labyrinths
Apocalypses
Artifice
Cataclysms
Inmates
Sublevel 3
The Nullweb
Apocalypses
Artifice
Cataclysms
Inmates
Sublevel 4
The Voidsink Maze
Apocalypses
Artifice
Cataclysms
Inmates