It formed part of the King's Device programme to protect against invasion from France and the Holy Roman Empire, and defended the Portland Roads anchorage.

Shortly after its construction it was armed with eleven artillery pieces, intended for use against enemy shipping, operating in partnership with its sister castle of Sandsfoot on the other side of the anchorage.

During the English Civil War, Portland was taken by the Royalist supporters of King Charles I, and then survived two sieges before finally surrendering to Parliament in 1646.

Fresh concerns over invasion led to the War Office taking it over once again in 1869, but the castle was not rearmed and was instead formed as accommodation for more modern neighbouring fortifications.

[1] Portland Castle was built as a consequence of international tensions between England, France and the Holy Roman Empire in the final years of the reign of Henry VIII.

Traditionally the Crown had left coastal defences to the local lords and communities, only taking a modest role in building and maintaining fortifications, and while France and the Empire remained in conflict with one another, maritime raids were common but an actual invasion of England seemed unlikely.

[2] Basic defences, based around simple blockhouses and towers, existed in the south-west and along the Sussex coast, with a few more impressive works in the north of England, but in general the fortifications were very limited in scale.

[7] In response, Henry issued an order, called a "device", in 1539, giving instructions for the "defence of the realm in time of invasion" and the construction of forts along the English coastline.

[12] In 1545, Mervin was replaced by John Leweston, who also took on responsibilities as the Lieutenant of Portland, along with a larger garrison of 13 men, paid for out of the proceeds of the recent Dissolution of the Monasteries.

[17] A 1623 survey reported that the castle was equipped with three culverins, nine demi-culverins and a saker, but that the fortifications had suffered badly from sea erosion and required extensive repairs.

[19] It was used to defend the Portland Roads during the First Anglo-Dutch War of 1653, seeing action in a three-day long naval battle between English and Dutch forces.

[19] When Charles II returned to the throne in 1660, he reduced the garrison to its pre-war levels but repaired the fortifications in the light of the continuing Dutch threat; in 1676 the castle was equipped with 16 guns.

[24] Following the final defeat of Napoleon, the castle was disarmed and leased to John Manning, a Portland churchman, who converted it from a fortress into a private house.

[38] The heart of the castle is the keep, 120 feet (37 m) across, comprising a central tower with two wings on either side and a gun battery to the front, together forming an unusual, fan-shaped design overlooking the sea.

[1] In the centre of the ground floor is the octagonal great hall, now fitted with large Victorian windows, which would have originally providing living space for the garrison.

[50] The current Governor's Garden was created in 2002 by the horticulturist Christopher Bradley-Hole, as part of a wider programme of work across English Heritage properties.