Beneath Römerplatz, between the Friedrichsbad and the Caracalla Therme, are the surviving ruins of the Roman imperial baths built in the 2nd and 3rd centuries AD. The complex was uncovered during the construction of the Friedrichsbad in the 1840s and has been left in place, accessible by a short underground walk.
What you see is the hypocaust system — the network of brick pillars beneath the floor that channelled hot air to warm the rooms above — and fragments of the bathing pools themselves. A small archaeological display explains the layout. It is a brief visit but it makes a useful introduction before crossing the square to one of the modern baths. The same water still pours up from the same springs.