Thorne Island commands the entrance to the anchorage of Milford Haven and access to the former Royal Dockyard at Pembroke Dock. A proposal was made to fortify the island in 1817, but it was not implemented. In the 1850s, there was growing concern about the increasing strength of the French Navy and the expansionist policy of the Emperor Napoleon III. Work started on the existing fort at some time after 1852 and a date of 1854 is carved above the entrance. The fort is an irregular polygon in plan and was designed with a seaward facing battery for five RBL 7 inch Armstrong guns and four 68-pounder guns, all mounted en barbette (i.e. in an open mounting, firing over a parapet). The landward side of the fort consists of a defensible barracks, with a loopholed parapet overlooking the entrance.
The fort was converted into a hotel in 1947 and was sold in 1999. In 2001 it was owned by the Von Essen hotel group who intended spending four million pounds to reopen the hotel with a five star rating and a cable car to allow access from the mainland. It has been largely abandoned since 2001 but in May 2017 it was purchased by the current owner who intends to undertake a project of restoration.
The fort became a Grade II* Listed Building in 1996, because it was "a well-preserved fort of Palmerston's Haven defences"
Divers recognise over twelve wrecks that are worth diving in the area of the island but of interest is the sailing ship that sank in 1878. In that year, 27 people were rescued from the sailing ship Loch Shiel which had run into rocks off the island. Two Angle lifeboat crew members and the honorary secretary received silver RNLI medals.
The rescue is particularly noteworthy as it is described as Wales' "Whisky Galore". The Loch Shiel was carrying goods from Scotland to Adelaide and included gunpowder, beer and cases of Glasgow whisky. Much of this was never recovered. Some of the bottles are still amongst the wreck which are described as "undrinkable", but many and much of the cargo was only partially recovered by the customs men.
Divers still investigate the wrecks off the island and they have found full beer bottles that came from the Loch Shiel.