The tunnel is long, dark and cut through solid sandstone, a testament to Victorian engineering. It opens out into a series of sunken, zig-zag trenches that in turn lead to a set of enormous circular gun emplacements. A century ago this historic military fort, which commands spectacular views of Sydney's harbour, rang to the clatter of pith-helmeted colonial troops in hobnailed boots.
Now the soldiers have been replaced by a modern army of carpenters, masons and electricians who are restoring the fortifications of George's Heights to their former glory.
The fort is one of seven former military sites dotted around the harbour that are being renovated by the Sydney Harbour Trust and returned to the public as parks and heritage sites. They occupy headlands and peninsulas with breathtaking views of city centre skyscrapers, the Opera House, beaches and bush land.
They date back to a time when Sydney, as an outpost of the British empire, feared attack from first the Russians and then the French. As the historian Geoffrey Blainey noted: 'An armed raider could emerge from the Pacific, enter the harbour at night, and do immense damage at a time when Sydney did not even know that Britain was at war with another sea power.'
From the 1870s, the colonial administration embarked on a frenzy of building fortifications. In 1891, nearby Chowder Bay became the base for the Submarine Mining Corps, a select unit whose job it was to maintain a line of underwater explosive mines across the harbour. They would be detonated under any attacking ships.
The advent of aircraft and long-range naval guns made the mines obsolete, and the corps was disbanded in the 1920s. But it was not until 1997 that the Australian military left the site for good, and at other locations it has clung on for even longer. Now the attractive, colonial-era wooden buildings are being diligently restored. A tiny brick shed which was once used to store gunpowder has just opened as a compact cafe.
'We deal with a different kind of powder now,' said Steve Dyer, busily grinding another batch of beans to make cappuccinos. 'It's great that these sites are being opened up to the public, rather than sold off to developers for millions of dollars.'
His customers enjoy one of the prettiest views in Sydney - a crescent of white sand, a turquoise bay and a backdrop of untamed bush land. Up the hill, a 19th-century artillery officers' mess is being converted into a restaurant.
After two centuries of hiding behind battlements and barbed wire, the forts are getting a new lease of life.