The Roman Baths, Bath, England
The Roman Baths is literally, a gigantic Bath that fills up the whole courtyard, it was mainly a common area for common people to pamper themselves or worship, meet, etc. It was rebuilt and turned into a museum since then. The museum did a great job of rebuilding a model of the city with many artifacts leaving clues to their way of life. In the architecture there was a distinct celtic influence I haven't seen in many other places.