The largest of these works was located at the rear of the Cartier Square Drill Hall, measuring 150 by 70 feet (46 m × 21 m), and was used for the storage of weapons, uniforms, and ammunition.

Both of these units are active components of Canada's Primary Reserve, providing trained volunteer soldiers to augment the Regular Army in peace and war.

For over 125 years, the Drill Hall at Cartier Square has been at the centre of military mobilization and training for soldiers recruited from the Ottawa-Carleton area, overseeing soldiers deployed for service in: During Ottawa Race Weekend each year, Cartier Square Drill Hall is converted into a satellite site of The Ottawa Hospital to provide medical services for runners.

[4] A statue dedicated to Privates William B. Osgoode and John Rogers, who were killed during the Battle of Cutknife Hill on 2 May 1885 during the North-West Rebellion, was moved to Cartier Square Drill Hall in 2006 from Confederation Park.

Originally constructed in the 19th Century for the officers of the Foot Guards, the mess was the scene of Sir Sam Hughes' (the Minister of Militia and Defence at the time) declaration of war by Canada in 1914.