Queencliff's fort was first used as a coastal defence in 1882 to protect Melbourne from a feared Russian invasion. It remained a base until 1946, and was then used as the Army Staff College until late 2012; today it functions as the Defence Department's archive. Tours take in the military museum (not always accessible), the magazine, cells and the twin lighthouses (including the iconic black lighthouse). It's a defence area, so bring ID for entry and arrive 10 minutes early.
A separate operator, Geraldine (a sixth-generation local with plenty of tales; 0410 512 472), leads tours (adult/child/family $15/7/35) midweek at 11am during the non-holiday period.