Harmesh Pooni, 42, who has the only set of keys, has become a virtual squatter on the 138-year old structure, and is stopping anyone landing as part of a legal dispute.
He has plastered notices on the windows – former gun turrets – warning he still owns historic No Man's Land Fort, and trespassers may be prosecuted.
Mr Pooni has also overturned tables and benches on the fort's two helipads and roped them down to stop estate agents flying in.
His antics are scuppering estate agents' plans to sell the £4m property.
Mr Pooni, who says he bought the converted stronghold in 2004 for £5m, is going to the High Court after it was seized by administrators.
'The fort is mine, and no-one else has the right to sell it,' he said.
'Even if they got an order to evict me, I shall carry on fighting in the courts.
'I have been staying at the fort in order to protect it and ensure no one boards and changes the locks. You could say I am the king of this castle.'
When Mr Pooni bought the fort he rented it out for £25,000 a day to corporate clients and events firms.
But the venue was shut down by health officials after the bacteria responsible for Legionnaire's Disease was found, and Mr Pooni struggled to pay his financial backers. Then his backers, Lexi Holdings, went bust owing £100m.
In a complicated legal fall-out, Lexi's administrator, KPMG, claims it owns the fort, and has put it on the market.
But Mr Pooni claims he was and still is the owner.He said: 'It's very spooky at night. I sit up in the lighthouse and look at the lights on the shore and hope the legal situation can be resolved in my favour.'
A judge has so far refused to grant an eviction order, and the matter is going to the High Court.
A KPMG spokesman said: 'We are currently undertaking legal proceedings to address the situation. We are unable to comment further.'
A spokesman for estate agent Knight Frank said: 'We can't comment, except to say we are still actively marketing the fort.'