Plaça del Rei
Time Out says
This well-preserved medieval palace square is home to the Museu d’Història de Barcelona and includes some of Barcelona’s most historically important buildings: the Escher-esque 16th-century watchtower (Mirador del Rei Martí) and the Capella de Santa Àgata, which houses the stone where the breasts of Saint Agatha were allegedly laid when the Romans chopped them off in Catania. Parts of the palace are said to date back to the 10th century; there have been many remarkable additions to it since, notably the 14th-century Saló del Tinell, a medieval banqueting hall that is a definitive work of Catalan Gothic. It's here that Ferdinand and Isabella are said to have received Columbus on his return from America.