 Becoming known as Athlone Castle, this 12-sided donjon dates from the 13th century.Other parts of the castle were largely destroyed during the Siege of Athlone and the external defences were subsequently rebuilt and enlarged.The currently visible battlements and cannon emplacements were installed to prevent a French fleet from sailing up the River Shannon and establishing a bridgehead in Lough Ree (likewise south of Athlone at Shannonbridge, near Clonmacnoise).