The Merrickville Blockhouse
When construction of the Rideau Canal began, Merrickville was already an established village and was considered to be a logical target for an invader. Consequently Colonel John By urged strong measures for the protection of the lock station. The result was this blockhouse, built in 1832-33, the largest on the Rideau Canal and the second largest surviving in Canada. It still resembles its early description as "a good blockhouse, the basement and ground floor being of stone, and the upper storey of wood covered with tin, the whole surrounded by a ditch".
Plaque via Alan L. Brown's site Ontario Plaques. Full page here.