The Baths of Caracalla
These are the best preserved and most grand baths from the ancient Romans. They were built between 212 and 216 AD and could supposedly hold up to 12,000 bathers at a time. Some places say 2-3K, the signs there said 8K… but the baths themselves included gymnasiums, a library, etc., and after being there, I think the 12K figure for the thermae area itself is more correct.