The hypocaust was an underfloor heating for the baths, so the floor was on top of the piles of stone, with the hot air flowing through underneath. This was first constructed as the winter quarters for a roman legion and apparently the baths had an outdoor pool "most unusual for baths in England, and which apparently proved unpopular" - can't imagine why!
A great three days in Shropshire, but we've worn ourselves out and need to get back to Dublin to rejuvenate. Hopefully the kids haven't made too much of a mess...