In 1880, Brisbane had fewer than 100,000 people but an annual trade worth over four million pounds, and it felt exposed. Australia belonged to the British Empire, but rival powers were carving up the Pacific, and Brisbane lay just three days' sail from the French naval garrison at Noumea. Moreton Bay was scattered with islands where a hostile fleet could shelter. On the advice of two eminent British military engineers, Sir William Jervois and Lieutenant-Colonel Peter Scratchley, the Queensland Parliament approved a fort at Lytton in 1878, and Scratchley designed it: a pentagonal earthwork garrison hidden behind embankments and protected by its moat. The fear was specific and named. Russia and France were the bogeymen, and the whole structure was an answer to the question that haunted every far-flung colonial port: what if they came for us?

Fort Lytton's defence began as a remote-controlled minefield strung across the river mouth, backed by four heavy muzzle-loading guns. The real ingenuity came later. By 1888 the fort had two six-inch breech-loading "disappearing guns," mounted on hydro-pneumatic carriages that hauled the barrel up to fire over the rampart, then dropped it back below the parapet about twenty seconds later, out of an enemy's sight and line of fire. Quick-firing Hotchkiss guns were added soon after. The minefield was closed in 1908, but the disappearing guns kept watch until 1938. A passing ship would have seen only an innocent green bank, with no hint of the heavy ordnance that could rise from behind it, fire, and sink from view before the smoke had cleared.

For most of its life, the fort's real work was training. For its first forty years it was the main base for Queensland's reserve soldiers, ordinary men who turned out each year for the famous "Easter Encampment," up to 5,000 of them pitching tents on the flats around the fort. Thousands trained here before shipping out to the Boer War, the First World War and the Second. These were not professional warriors but clerks, labourers and farmers learning the trade of arms for a fortnight at a time, then going home. The grainy old photographs of soldiers and horses massed at Lytton are a portrait of a colony, and later a young nation, building its army out of volunteers, one Easter at a time.

Fort Lytton's most dramatic role came in the Second World War, when Brisbane became a major Allied submarine base and 89 submarines were dry-docked there over three years. The fort anchored the river's defences: an anti-submarine boom net stretched across the channel, opened and closed by winches at the fort and on a permanently moored ship, the HMAS Kinchela, with heavy guns trained on the gap. After the war the guns fell silent, but the signals station kept listening, even gathering intelligence on the 1965 upheaval in Indonesia before it finally closed that year. The site was handed to an oil company, which transferred the historic fort back to the state in 1988. In 1990 it became Fort Lytton National Park, its ramparts now grassed and quiet, a fortress that fulfilled its purpose precisely by never having to prove it could.