That whole place is amazing. All the little nooks and crannies and caves and underground ponds. The whole Regalia is amazing.

And no, it's not a well. At the very bottom is a marble floor with a compass and they used to do occult - style ceremonies there. The spiral staircase down has a lot of different nooks that held candles and the ceremony participants would find the stairs to the bottom. There's another entry at the very bottom that leads out to the back of a waterfall and that's where the " special participants " would come and go