Miróbriga, Roman archaeological site in Santiago do Cacém, Portugal

Miróbriga is a Roman city on Castelo Velho hill, 245 metres high, one kilometre north-west of Santiago do Cacém. Its forum, with temples to the emperors and Venus, occupies the hilltop; two L-shaped bath buildings with hypocaust heating stand in a valley, and a single-arch bridge crosses a road east of them. The circus, 370 by 75 metres, lies about 500 metres south.

Occupation dates to the Iron Age, from at least the 5th–4th century BCE and possibly the 9th century BCE, when a fortified settlement covered the Castelo Velho hill. Roman occupation began around the second half of the 1st century CE, when the forum, baths and paved roads were built. The town was not abandoned after the 5th century: a reduced population remained around the Chapel of São Brás into the Early Middle Ages.

The name Miróbriga combines the Celtic suffix -briga, as Pliny the Elder noted when he called its people Mirobrigensis qui celtici cognominantur. The circus, not a hippodrome, was a 370 by 75 metre arena for chariot and horse races, with a central spine and a triumphal arch at its south entrance. The two bath buildings, Western and Eastern, each had a massage hall, gymnasium, changing room and communal latrine.

The ruins lie about one kilometre north-west of Santiago do Cacém, reached by a road off the town; the circus is a further 500 metres south of the main complex. Paths follow the Roman paving and are uneven, and the site is open to the public as part of the Alentejo and Algarve Archaeological Itineraries. The baths sit in a valley below the Castelo Velho hill, so the visit involves slopes.

The circus, about 500 m south of the centre, is the only one in Portugal whose entire ground plan is known: a 370 by 75 m rectangle with curved seating at the north end, segmented seats at the south, a central spine with posts at both ends and a triumphal arch at the south entrance. It hosted chariot and horse races.