Those little pillars were supposed to hold up a floor, which no longer exists. In those days, slaves will have to burn wood or whatever to produce hot air which they fan underneath the man-made floor (the one that no longer exists) so that ppl standing on the man-made floor would have nice warm feet. The Romans also had this whole duct system to make sure the hot air transfer its heat thru out the complex. Once again, very smart of them.