The Foundations of Decay begin by descending a long interstitial ladder from Sewers - Backrooms leading into the northwest of the Foundations of Decay.
For note the Sewers - Backrooms are an intermediary set of rooms connecting mostly to the Sewage Treatment Plant.
On the opposite(NE) side of the room is the entrance to Tunnel 4.
Heading up the malfunctioned elevator shaft in Tunnel 4 brings the player and party out onto an extension of the Foundations: a towering cliff's edge.
This second floor area also connects to 2 separate rooms in the yet further Crypts area. The further west of the 2 Crypts' Nooks has an elevator the player can activate to either return back down to the Old Town - Slums or ride up into the Church of Alll-mer lower levels.
Gameplay
There are little enemies in this area, although there are two difficult encounters. After reaching a forking path where a Ratkin and blue moths can be seen, proceeding to follow the fleeing ratkin will result in being ambushed by the Ratkin gang in their den full of loot, while choosing to follow the moths will result in an encounter with Moonless in front of the passage to Tunnel 4.