This building is famous for its architecture, particularly the stunning fan vaulting of its Gothic ceiling. I began this blog with a picture from another corner of this room. You can see the grating in more detail if you follow this link.
This circle in the pavement is a mystery to me. It was at the top of the earthen mound that is the oldest part of Warwick Castle. The mound was built in 1068 "on the orders of William the Conqueror," according to a sign at the castle. The top of the mound was not large - maybe the size of a good-sized livingroom, but open to the air with low walls on most of the sides and no roof. It was like a garden terrace, and this construction was on the rain-drenched pathway between the entrance and one of the low walls. I'm trying to remember the size. I think it was a bit larger than a typical manhole cover. In fact, it may have been as much as two feet across.