FOR the price of a one-bedroom Sydney apartment, you could buy an entire island.

Stack Rock Fort, a heritage-listed fortification off the coast of Milford Haven in Pembrokeshire, southwest Wales, has gone on sale for £400,000 ($706,400), with the seller describing it as an “amazing opportunity to own a real piece of history”.

The fortification was built between 1850 and 1852 and upgraded in 1859 with a new building that completely encased the original gun tower. It was one of a chain of forts designed to defend Milford Haven and the Royal Naval Dockyard at Pembroke Dock against an invasion from France under Napoleon.

“The island is currently uninhabitable but has unbelievable scope for development,” the real estate listing reads. “Previous owners have inquired about the potential to turn the fort in to a commercial residence with links to the main land.”

It was built in 1850 to defend against the French. Stack Picture: Wales News Service/australscope

The three-storey structure once housed 150 men. Picture: Wales News Service/australscope

It was abandoned in 1918 and has changed hands several times. Picture: Wales News Service/australscope

The fort once accommodated nearly 160 men and housed massive anti-ship guns. It consists of three floors with spiral staircases between each. It was abandoned in 1918 and sold for £60 in 1932. It changed hands again in 2005 for £150,000.

In 2013, owner Gary Phillips raised security concerns after a suspected break-in. Mr Phillips said a friend had sailed past and noticed the door was open and appeared to be damaged, but police took no action.

“The officer told me that because of the rusty condition of the door fittings the wind must have forced it open, but that didn’t sound right so I went to the fort this morning to see for myself,” he told The Pembrokeshire Herald.

There was an industrial grade lock and hasp. The door is still on its hinges and there is evidence that the lock has taken a hammering and the hasp has been hacksawed in two.

“Whoever broke in must have had the right kit for the job with them and made a hell of a noise — the place was locked up like a fortress. I just don’t understand how it could happen right under their noses.”

frank.chung@news.com.au