Fort Mississauga
This tower and earthwork are all that survive of the barracks, guardroom, and cells of Fort Mississauga. Built between 1814 and 1816 to replace Fort George as the counterpoise to the American Fort Niagara immediately opposite, it was garrisoned until 1826. Repaired and rearmed following the Rebellion of 1837, it continued to be maintained until 1854 in response to border disputes with the United States. It was manned during the tense years of the American Civil War and the Fenian scare of 1866, but by 1870 it was no longer considered of military value.
Plaque via Alan L. Brown's site Ontario Plaques. Full page here.