'Bomb-proof estuary gun emplacement' at No 1, The Thames, is 1km offshore and can only be reached without a boat at low tide

Phrases like "character property", "unique location" or "genuine investment opportunity" do not really measure up to the job. Abandoning traditional estate-agent speak, Nigel Day said fervently: "I'm not going to lie to you, it's an absolute monster."

The Grain Tower Battery, a Victorian fort with attached hideous 20th-century red brick barracks block and concrete gun tower, lies off the Isle of Grain where the river Medway meets the Thames estuary. It is completely surrounded by water at high tide.

It is somewhat of a bargain at a guide price of £500,000, cheaper than most of the one-bedroom flats on Day's books.

The address of the property is as imposing as the views: No 1, the Thames – almost grand enough to make up for minor inconveniences such as the lack of electricity and running water.

It could also be a seriously awkward place to run out of milk. At low tide it can be reached by a slimy brick causeway, but owners will need a firm grasp of their tide tables: at high tide boat or helicopter are the only ways in or out.

"It's fabulous, but it is pretty scary," Day said. "I thought I was OK on heights, but there are bits of it where I was clinging on to the walls: you realise if you fall you aren't coming back in a hurry.

The fort could be used as a hotel, nightclub or casino, Riverhomes suggests. Photograph: Wt Architecture/PA

"It's huge. You do get the sense that it really is bomb proof – but there are places where your legs turn to jelly a bit.

"The interest has been absolutely phenomenal; we've had people talking about turning it into a hotel, a home, a recording studio or a club."

The oldest part of the battery was built in 1855, part of a chain of coastal defences built because of the continuing dread of invasion from the sea after the Napoleonic threat half a century before. Although guns were installed at various anxious points in the 20th century, like most coastal forts it was rarely fully armed or garrisoned.

The fort has not been entirely abandoned, as it has been covertly visited by various urban explorers. Potential purchasers might like to bear their reports in mind. "Beware," one recent group warned. "Without a ladder it would be an extremely risky climb with the very real potential for a fall from a very considerable height on to jagged rocks and concrete."

The owner, Simon Cooper, is a builder who was working near the estate agent's office in Putney. Cooper bought the fort from the Crown ten years ago for an undisclosed price, intending to restore it. He told the BBC: "It didn't work well as a home."

Day recalled: "He popped in one day and said 'I don't suppose you can help me sell my property?'

"We asked where it was and he said: 'Sheerness way.' I told him politely we only cover London riverside property and he said: "Well, the address is No 1, Thames" – and I just couldn't resist."