The Square Tower is one of the oldest parts of the fortifications of Portsmouth, England. The tower was built in 1494 as part fortifications and as a home to the governor of Portsmouth. In 1584 it was converted to a gunpowder store, the governor moving residence next to the Garrison Church. At the time of the royalist surrender of Portsmouth at the end of the Siege of Portsmouth during the English Civil War 1200 barrels of gunpowder were stored in the tower.