Closer inspection reveals that the bird is a strangely contorted metal duck impaled on the weather vane - a weird, century-old memory of the oil magnates who once lived here. And a little research reveals that this rambling pile, dating from the early 18th century and overlooking Greenwich Park, just across the Thames from Docklands, is not even a real castle.
Alistair Wilson, a local lawyer, fell in love with the place 15 years ago, when it was on sale for pounds 100,000. This was long before he had achieved the earnings of a top barrister, so the price was out of his reach.
Then an architect friend worked out how to split the building between several buyers, and a gaggle of friends expressed interest in buying jointly. 'But they got cold feet,' Mr Wilson recalls. Undaunted, he advertised for other pioneers and received three replies. So the house was quartered - an experiment that was so successful that three of the quarters still have their original
Despite the castle's Grade I listing, the division was relatively easy. 'We just blocked up a few doors and put in one dividing wall,' says Mr Wilson.
That he has a passion for DIY is not obvious: the hand-stencilled gold crests in a black-floored lavatory could be a couple of centuries old, while the garden pavilion looks as though it was standing before the house was built.
This hands-on approach would have impressed the original owner, Sir John Vanbrugh. There is no evidence that he ever tried his hand as tinker or tailor, but in a crowded life he did manage soldier and spy as well as playwright, theatre manager, public servant and, most famously, one of Britain's great architects.
Castle Howard - the Brideshead of television fame - is his creation. So are Blenheim and Stowe, two of the most potent symbols of a grand age of country living. They provided his passport to succeed Sir Christopher Wren as Surveyor of the Royal Greenwich Hospital in 1716 (although it probably helped that his brother was secretary to the hospital committee). A couple of years later he started building his own home.
Almost three decades earlier, Vanbrugh had been locked up for several years in the Bastille as a spy. Instead of burying the experience, he drew from it the inspiration to recreate the French prison in miniature. Choosing a site high on the eastern edge of Greenwich Park, where he could see both the hospital below and London on the horizon, he spent two years building what came to be called Bastille House.
It was the third home he had created for himself, but the only one to survive. He lived there little longer than the time he spent in the Bastille. In 1726 he died - an event not unrelated to marrying a young wife, according to court gossip.
His last monument changed identity frequently over the years, as well as changing its name. 'Apparently it was once the local home for fallen women,' says Mr Wilson. The Duckham family was living there by the turn of this century, bequeathing the rooftop duck. The RAF Benevolent Fund turned the place into a school before it passed to the Blackheath Preservation Trust. The Wilsons have not erased the chapters of its history and famous RAF names such as Cheshire, Gibson and Trenchard still mark out rooms fitted with Vanbrugh fireplaces.
The estate agent John D Wood is now looking for new castle sharers to take over stewardship of the monument. Mr Wilson is hungry to indulge his DIY passion on a new challenge. 'Perhaps an old barn or country house that needs lots of work,' he says. He and his wife Susie, also a barrister, have already spotted something suitable - an old watermill with all the original components. 'It may even take 15 years to get it back into working order,' he enthuses.
If the Wilsons manage to sell their share of Vanbrugh Castle at the asking price, it would add to this building's living history a fascinating footnote on today's property market. Fifteen years ago Mr Wilson managed to beat the vendors down to pounds 87,000 for the whole castle. Today he is asking pounds 425,000 for just a quarter of it.
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