A surprisingly substantial chunk of Roman brickwork — three storeys of arched walls in places — surviving in the middle of the city between the Campo dei Miracoli and Piazza dei Cavalieri. The name is medieval folklore; the baths are actually Hadrianic, built around 100 AD, and were probably the public baths of the Roman colony of Pisae.
They are unfenced and you can wander right up to the walls. The brickwork shows the characteristic Roman pattern of small bricks laid in alternating courses with the occasional terracotta tube where the hypocaust ran. A useful five-minute reminder that Pisa was a thousand years old before the Leaning Tower was built.