Fort Mississauga is a small British fortification at the mouth of the Niagara River, built after the War of 1812 and now a National Historic Site that sits somewhat incongruously within the grounds of the Niagara-on-the-Lake Golf Club. The round brick tower — a rare surviving example of a Martello-style tower in Ontario — stands at the lake's edge and can be visited for free.
It lacks the interpretive infrastructure of Fort George but offers something Fort George doesn't: a quiet, somewhat forgotten atmosphere and excellent views over Lake Ontario and the river mouth. The site is easy to combine with a walk along the lake foreshore. Access is via the golf club entrance on Front Street.