Portsmouth Harbour is a 1,264.2-hectare (3,124-acre) biological Site of Special Scientific Interest between Portsmouth and Gosport in Hampshire. It is a Ramsar site and a Special Protection Area. It is a large natural harbour in Hampshire, England. Geographically it is a ria: formerly it was the valley of a stream flowing from Portsdown into the Solent. At its north end is Portchester Castle, of Roman origin and the first fortress built to protect the harbour.