Modern Beja stands on the site of Pax Julia, a significant Roman city that served as an administrative hub for the interior of the province of Lusitania. Walking through the old town, you are walking over Roman streets — remains turn up regularly during construction work, and the city has a long history of archaeological discovery.
The best surviving Roman material is in the Museu Regional, particularly the mosaic floors recovered from wealthy Roman villas in the surrounding countryside. Some of the mosaics are outstanding — intricate geometric and figural designs in excellent condition. The Roman walls of the city are partially visible in sections of the medieval defences; look for the contrast between the larger Roman stone blocks and the smaller medieval masonry.
For a more systematic view of the Roman city, the museum's ground-floor displays give a good overview of what Pax Julia looked like at its height.