Established in the late 1870s, Fort Nepean played an important role in Australia's military defence. Located at the far western tip of the national park, it retains relics such as gun emplacements, ammunition magazines, barracks tunnels and a bomb-proof room. The site boasts the remarkable claim to fame as being the spot where the first allied shots were fired in WWI and WWII – the guns used in the conflicts are on display here.
To reach the fort it's a 2.6km walk from Gunners Cottage car park, or you can jump on the shuttle bus (adult/child $10/7.50) that travels between the Quarantine Station and the fort; this operates every 30 minutes between 10.30am and 4pm.
For the record, the story of the first shots fired during the wars is probably less interesting than it sounds: in WWI a cannon was fired at a German cargo ship SS Pfalz on 5 August 1914 after it failed to stop (the crew were duly detained in Portsea); in 1939 a warning shot was fired towards a ship that had failed to announce itself (literally hours after the declaration of WWII), but it later turned out to be an Australian freighter.