Sydney It may sound improbable, but Sydney's Fort Denison was built as a deterrent against Russian invasion in the 19th century. The Crimean war was in full swing and Australia, as one of the farthest outposts of the British Empire, feared Russian expansionism in the South Pacific. '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,' Australian historian Geoffrey Blainey wrote. The feared tsarist invasion never happened, but the sturdy little island fortification remains, crouched in the middle of the harbour, just north of the Botanic Gardens and Garden Island naval base. Fort Denison now faces a new threat to its existence. It is under assault from sea spray, the pounding of waves and the prospect of rising sea levels caused by global warming and the melting of the ice caps. This week is the convict-built fortification's 150th anniversary. To coincide with the event, authorities have announced a major renovation project. Costing A$1.5 million (HK$10.38 million), it will take three years to complete and entail a team of stonemasons, archaeologists and architects shoring up the fort's foundations and walls. A picturesque landmark of honey-toned sandstone, it is Australia's only Martello tower and one of only two in the southern hemisphere. Britain built Martello towers as a line of defence against the threat of invasion by Napoleonic France early in the 19th century. 'We don't want this icon to become the first victim of climate change,' state Environment Minister Phil Koperberg said. The Intergovernmental Panel on Climate Change has warned that sea levels could rise by nearly a metre by the end of the century. The fort has undergone repairs before, but not on the scale of this new project. Remedial work will include replacing badly eroded sandstone blocks and researching ways of better protecting the stone from wave erosion. 'The damage is quite serious,' archaeologist Cath Snelgrove said. 'It's an extreme site, right in the middle of the harbour. It's built on bedrock, so there's always going to be some saltwater creeping in. But we can take the salt out of the interior walls, and on the outside walls make sure that the mortar is in good condition.' The tiny island has a small museum and a cafe. The gourmet sandwiches and chilled Chardonnay on offer are a far cry from the island's gruesome origins. Early convicts knew it as Pinchgut Island, for the starvation rations given to recalcitrant prisoners who were banished there for days or weeks at a time. The unannounced arrival of two American warships in the harbour in 1839 spooked the fledgling colony and made it painfully aware of how distant it was from the protection of the mother country. 'It's true they came here in the character of friends, but had their intentions been hostile we neither possessed the means to prevent their entrance in the first instance, nor to have protected ourselves from the subsequent consequences,' the Gazette newspaper noted at the time. Ironically, the fort was obsolete by the time it was completed. 'The 32-pounder guns they winched in were, by then, no longer state of the art,' Ms Snelgrove said. 'And it was realised that firepower was needed at the entrance to the outer harbour, not right in the middle of it.' Fort Denison never had to withstand a Russian fleet. Whether it survives rising sea levels is another question.