The fort in 2012 Stack Rock Fort viewed from above, 2021

A fortification at Stack Rock was first proposed by Thomas Cromwell in 1539 to protect the waterway, although this would not actually come to fruition at the time. Similar proposals were made in 1748 when Lewis Morris carried out a survey of Milford Haven, reporting on shipwrecks and navigation and recommending that a small fort be built here. Another survey followed in 1817. The Royal Dockyard at Pembroke Dock was felt to be in need of defence from the sea,[1] The result was a tower mounting three guns, similar to the two towers in Pembroke Dock.[2] The tower was oval in section, with a maximum diameter of 55 feet (17 m) and a maximum height of 30 feet (9.1 m).[3] Construction commenced in 1850, being completed in 1852.[4]

In 1858, the first appearance of the French ironclad Gloire rendered the tower and its guns obsolete, so it was decided to completely surround it with a new battery. The original design was for two decks each of 19 casemates and 16 open air guns above, 54 guns in total, and built mainly of limestone. However, after the foundations had been started in 1861, the design was amended to have stronger granite piers supporting iron gun shields. The basements and magazines had been completed in 1863, when the design was altered again, for 16 large guns on a single floor of casemates in the front and seven smaller guns on two floors in the gorge, the rear of the fort facing into the Haven. The top floor was built as accommodation intended for 4 officers and 152 men. On the roof of the fort, three gun turrets each with two huge RML 12-inch 25-ton gun were planned, but never installed. This scheme was finally completed in 1871.[2] The usual peacetime garrison was one officer and thirty men.[4] Disarmed in 1929, it was first placed on the market in 1932 and sold for £160.[4] In 2005 it was sold once more for £150,000.[5]

Concerns over the security of a nearby LNG jetty were raised in 2013, following the discovery of potential trespasser activity.[6]

The fort was sold again in 2020 to a private owner for an undisclosed amount. Land Registry records show[7] that it was sold in January 2021 for £191,000 to community interest company (CIC) Anoniiem.[8]