 During this period a tower, used either as a watchtower or bell tower and containing a spiral staircase, was constructed at the southwest corner of the cella, and vaulted tombs were built beneath the Parthenon's floor.In 1456, Ottoman Turkish forces invaded Athens and laid siege to a Florentine army defending the Acropolis until June 1458, when it surrendered to the Turks.The Turks may have briefly restored the Parthenon to the Greek Orthodox Christians for continued use as a church.