It looks like the archway is filled with holes for candles. Lighting for meetings could not have been as trivial a task as it was for the researchers, who seem to have been able to toss down some cheap super bright LEDs(?).

An hour of good lighting in a subterranean cave in those days must have cost a fortune. (Per Jane Brox, anyway...)

https://www.amazon.com/Brilliant-Evolution-Artificial-Jane-B...