Its location bordering Bell Road, Trollope Street, and Ahern Avenue is adjacent to the Halifax Common and immediately west of Citadel Hill, a National Historic Site from which the school derives its name.

On 31 March 2005, the Halifax Regional School Board announced the new name of the school, "Citadel", which was selected in a vote among other suggestions including "Halifax Central", "Willow Tree" (for the nearby traffic interchange of the same name), "Patrician Elizabethan", "Ahern Bell", "Gardens North", "Peninsula", "Garrison", as well as numerous famous Haligonians including Vince Coleman, Anna Leonowens, Robert Stanfield, Portia White, Samuel Cunard, Robert McCall, Joseph Howe, Edward Cornwallis, Richard Bulkeley, and Alexander Graham Bell.

The school began operation in September 2007, and was officially opened by Premier Rodney MacDonald and Minister of Education Karen Casey in November of that year.

During the 2011 Canada Games, Citadel High served a competition venue for wheelchair basketball and table tennis.

QEH and St. Pat's offered Pre-IB courses in 2006–2007 to Grade 10 students preparing for the IB program at Citadel High.

The two large gymnasiums at the school are complemented by outdoor basketball courts and the numerous other sports facilities at the Halifax Common across the street.

The school contains over 60 teaching spaces: classrooms, laboratories for biology, physics, and chemistry, computer labs, a woodworking lab, family studies kitchen and sewing facilities, two art rooms, and a drama room adjoining the newly opened auditorium, the Spatz Theatre.

[17] The 450-square-metre (4,800 sq ft) cafeteria is called the Portia White Atrium, named after a Nova Scotian singer who achieved international fame and performed for Queen Elizabeth II, was featured on a postage stamp, and is the namesake of the Portia White Prize, awarded to established artists within the province.

Built-in furniture, cabinetry, wall paneling, and much of the other custom interior finishing was designed by Chandlers Millwork of Charlottetown[20] and, in line with the school's LEED certification, does not contain urea formaldehyde, commonly used in wood products.

[4] Sandstone banding integrated into the brickwork of the new school was salvaged from the facade of St. Pat's, which was removed earlier as the anchoring for the cladding had deteriorated and became a safety hazard.

A 20-foot (6.1 m) tall stainless steel tree, a sculpture by Dartmouth artist Dawn MacNutt, was installed in the main foyer in early 2008.

Entitled Together We Stand, it is suspended in the three-storey atrium at the axis of the building and is made of steel wiring with leaves of aluminized polyester woven in Switzerland.

The piece was originally located in the lobby of the Infirmary building at the nearby Queen Elizabeth II Health Sciences Centre until renovations there necessitated its donation to the school.