Also located inside Sydney Harbour and part of its National Park, Fort Denison was once a penal site and defensive facility that occupies a small island about one kilometre east of the Sydney Opera House. Formerly known as Pinchgut Island, it was open and used as maximum security prison from 1788. The fortress itself was built in 1857, with a distinctive Martello tower, to help protect the harbour from attack by foreign vessels. Fort Denison today is a museum, tourist attraction and a restaurant, as well as a popular venue for weddings and corporate events.