The Merrickville blockhouse is one of four blockhouses built on the Rideau Canal and the largest on the canal, with a fifty-foot-square stone lower level, three-and-a-half-foot-thick walls designed to resist small cannon fire, a pyramidal tin-sheathed roof to resist torching, an overhanging upper level for machicolated defense, and tapered loopholes that gave defenders a greater angle of fire. Although small defensible lockmasters houses were originally intended for various lock stations, Lieutenant Colonel John By, the Royal Engineer in charge of Rideau Canal construction, considered them insufficient and proposed twenty-two large blockhouses, but the Board of Ordnance rejected that plan because of the cost. In 1831, By began building four blockhouses on his own initiative, and they were completed by 1833. Merrickville was a particular concern as the Prescott road was improved and the wilderness buffer between Canada and the United States was reduced. The building could house fifty men in its barracks area and store ordnance, arms, and ammunition in its lower stone section. Built to defend the Rideau Canal from American aggression, it was never permanently staffed by the military, though during the 1837 Upper Canada Rebellion the 34th Regiment stayed there briefly while en route to Fort Malden.