So in 1804 work began on Fort Phillip. To stand on what was then Windmill Hill, the township's highest point, it would be in the shape of a hexagon, with stone walls several metres thick. However, in 1806, with only three of the six walls finished, all work stopped.

Two centuries later archaeologists have started digging on the site, now inside the grounds of Sydney Observatory, to learn more about the abandoned project and the concerns of the colonial rulers. When it came to the Irish, said Caitlin Allen, an archaeologist from the Government Architect's Office, "they were paranoid". Guns mounted on the fort's completed walls, she said, faced back towards the town. "That tells you they were probably more worried about an attack from inside the colony."

During an armed rebellion "they would be able to bring the garrison up from Dawes Point, and the other fortifications around the harbour, and into the fort to defend the town". Fort Phillip was designed to outlast any siege.

Last week Ms Allen's team made a big discovery, unearthing remains of a bomb-proof shelter that had been built into one wall. Besides storing gunpowder, it could have also provided cover during an attack. "Wonderful sandstone blocks with curved faces [tell] us it had a completely domed roof," she said. "We have also found the lovely flagstone floor of a small building that once stood in a courtyard that was in the middle of the fort." Other relics discovered include a handful of iron shot. Just six weeks after work started on Fort Phillip, Irish convicts launched the brief Vinegar Hill uprising at Castle Hill. In December that year King wrote to England, reporting that the "citadel is far advanced and when finished will afford the greatest advantages in resisting any attempt that may be made in the settlement".

But it was never finished. In 1848 one wall, which still stands today, was converted to serve as a platform on which a semaphore station and flagstaff were built. In the 1850s the rest was levelled for the construction of Sydney Observatory. Working on such "a substantial structure" from Sydney's earliest days was "a treat", Ms Allen said. She said people were welcome to watch the digging, during observatory business hours.

At the site, on Sunday, November 2, she will talk about her work as part of the Historic Houses Trust Sydney Open day. Tickets - $30 or $25 concession - can be booked on 8239 2211, also provide entry for the day to 50 sites around Sydney.