Fort Blockhouse is the final version of a complicated site. A blockhouse was first built on the Gosport side of Portsmouth harbour in 1495, with 5 guns. Henry VIII ordered it replaced with an 8 gun battery as part of his Device Forts in 1539. This had probably vanished by 1667 when Bernard de Gomme installed a 21 gun battery for Charles II. But in 1708 the fort was rebuilt on an irregular trace. Upgrading was done at the turn of the 19th century, and again in 1845, from which time most remains date. The site was considered obsolete by the 1859 Royal Commission on the Defence of the United Kingdom, and it was turned over to the Royal Navy, where as HMS Dolphin shore-establishment, it was the home of the submarine service for years.