The Fort of São Bruno is situated on the estuary of the River Tagus in Caxias, Oeiras municipality, near Lisbon in Portugal. It was built in 1647 and became operational in 1649 as part of the construction of a line of forts to control access to Lisbon, which stretched from Cabo da Roca on the Atlantic coast to the Belém Tower near Lisbon. The fort is well preserved, following its original design, and is considered one of the most attractive examples of maritime military architecture on the Portuguse coast. It presently serves as the headquarters of the Associação Portuguesa dos Amigos dos Castelos (Portuguese association of friends of castles).