 As with Charles Fort on the other side of the harbour, James Fort was occupied by Jacobite forces during the Williamite War in Ireland.It was captured however in 1690 by Williamite forces, after being damaged by an explosion of gunpowder stores.Though Charles Fort (the "new fort") was operated as a military garrison through to the early 20th century, James Fort (the "old fort") declined in use during the 18th century, and texts and maps describe it as a ruin by the 19th century.