Fort Blockhouse is a military establishment in Gosport, Hampshire, England, and the final version of a complicated site. It is surrounded on three sides by water and provides the best view of the entrance to Portsmouth Harbour. It is unique in two respects - firstly, it was built over a number of centuries. Secondly, it is thought to be the oldest fortified position in the United Kingdom that is still in active military use.
Following the burning of Portsmouth during the Hundred Years' War, money was set aside in 1417 to provide protection for Portsmouth Harbour. A blockhouse was first built on the Gosport side of the harbour in 1431 after authorisation by Henry VI. The defences were upgraded in 1495 and armed with five guns.
The blockhouse was replaced in 1539 by an eight-gun battery under the orders of Henry VIII after his divorce from Catherine of Aragon. The first firing of the guns is believed to have occurred during the English Civil War when the guns were aimed at Southsea Castle after Parliamentary troops had captured it. The aim was not good, however, and the cannonball landed in St. Thomas' Church in Old Portsmouth.
The original fort is believed to have disappeared 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; in the 1870s it became the headquarters of the Portsmouth Company of the Submarine Mining Engineers.
In 1905 Fort Blockhouse was turned over to the Royal Navy and, as HMS Dolphin, it became the home of the Royal Navy Submarine Service. Between the two world wars, the establishment expanded beyond the lines of the original Fort; there was further expansion in the 1950s, when the prominent submarine escape tower was built. In 1992, it was announced that the submarine fleet would be leaving HMS Dolphin and moving west to HMNB Devonport. The last submarine left Dolphin in 1994, although the submarine school remained until 1999. HMS Dolphin was formally decommissioned in 1998 and the base was renamed as Fort Blockhouse.
The site is open for tours in September each year as part of the Heritage Open Days scheme you are advised to check dates etc. With the closure of Royal Hospital Haslar to serving Service Personnel, Fort Blockhouse is now known as Support Unit Fort Blockhouse, and staff support many lodger units.