This omits the NW tower and includes a few rooms from the Torture Chambers and the Southwest Wing, First Story.

Rat Hole A19
A19 -- A17 -- A15 -- ?? -- B11
East of B11 -> B19
West of B11 -> Rat HQ

The rat hole in A19 leads, after a fashion, to the one in A17 and then the one in A15. Skipping a branching path (??) will lead to B11, which has E/W branches, one leading to B19 and the other leading to a Rat HQ.