When construction of the Rideau Canal began, Merrickville was already an established village and was considered by the authorities to be a logical target for an invader. Colonel John By therefore urged strong measures to protect the lock station, resulting in a blockhouse built in 1832-33 that was the largest on the Rideau Canal and the second largest surviving in Canada. It still resembles its early description as a good blockhouse, with a basement and ground floor of stone, an upper storey of wood covered with tin, and the whole surrounded by a ditch.