By today's standards, half a million pounds for a cavernous property with unrivalled views across the Thames might sound like a bargain.

A disused fort with the memorable address of Number 1, The Thames, has gone up for sale with a guide price of £500,000.

But there are some drawbacks - it can only be reached - without a boat - at low tide and requires a "complete renovation".

The unique building off the coast of the Isle of Grain in Medway, Kent, is described as a "bomb-proof estuary gun emplacement" which was build around 1855.

It was described as "absolutely vast", with room for up to seven bedrooms.

Estate agent Riverhomes said the original tower at the property - also known as the Grain Tower Battery - is " similar in design to the Martello towers which were first constructed as a defence against Napoleon in the early 19th Century".

Nigel Day, from the firm, which specialises in waterside residences, said he had not come across anything like it before.

"It's so different than anything we usually sell or rent," he said. "It's a 150 year old sea defence about a kilometre off shore.

"Once I heard the address I couldn't resist getting involved and helping find a buyer."

He said the property also has foreshore rights meaning the owner has control of the causeway that leads out to the building.

Riverhomes suggested the property could be transformed into a private residence, a hotel, a houseboat community, nightclub or casino, while one inquirer canvassed the idea of turning it into a recording studio.

The first viewings are due to start this weekend.

The current owner Simon Cooper told the BBC he bought the property from the Crown Estate around 10 years ago.

He said: "We agreed a price and I purchased it. It didn't work well as a home and plus the cost of doing it..."