Stretching from the Plaça del Rei to the cathedral are some 4,000sq m (43,000sq ft) of subterranean Roman excavations – streets, villas and storage vats for oil and wine, all discovered by accident in the late 1920s when a whole swath of the Gothic Quarter was dug up to make way for the central avenue of Via Laietana. The excavations continued until 1960; today, you can get to the labyrinth via the Casa Padellàs, a merchant's palace dating from 1498, which was laboriously moved from its original location in C/ Mercaders to allow the construction of Via Laietana.
Admission also gives you access to the Capella de Santa Àgata – with its 15th-century altarpiece by Jaume Huguet – and the Saló del Tinell, at least when there's no temporary exhibition. This majestic room started out in 1370 as the seat of the Catalan parliament and was converted in the 18th century into a Baroque church, which was dismantled in 1934. The Rei Martí watchtower is closed to the public. Tickets for the museum are valid for all seven MUHBA sites, including the monastery at Pedralbes and the Museu Verdaguer.