Related to comments on Where did Qyburn suggest that Cersei go?
In Game of Thrones S08E05, towards the end we see Cersei & Jaime in a cellar we have seen previously in the show, with the various-sized skulls of dragons.
Arya chased a cat in there, Varys was conniving there, I have a vague but indistinct memory Joffrey may have even taken Sansa there...
But where was it? What building/castle/keep is it under?