In 1798, the remains of Lisbon's Roman amphitheatre were discovered dug into the side of the hill, in what must once have been a very dramatic location just uphill from the Sé. It's been excavated and turned into a simple but very attractive museum.
Pátio do Aljube, 5 (off Rua Augusto Rosa) or Rua da Saudade; tel: 21 75 13 200;
Tram 28.