Completing the group of Military Compound buildings, this 1828 sandstone, wood and slate-roofed structure was designed by a Captain Graydon of the Military Engineers and housed various essential bureaucratic and security functions; including an orderly room and guard room (both facing the colonnaded porch), lockup cells, and a solitary confinement pit in the basement. The restored orderly room looks as it did in 1829 when it was occupied by the 1st Battalion of the Rifle Brigade. The guard room was home to the 12-man Main Guard, and a private in an infantry regiment of 1828-1869 could expect to be detailed for Guard duty at least twice a month. The room looks as it did in 1866, when the 15th Regiment used the muskets, uniforms and equipment on display. The rear cellblock originally contained seven cells - windowless with an air-hole of about 10 inches diameter in the outer wall. In very cold weather, the luckless inmates were provided with wooden plugs to reduce the winter blasts. In 1847, the cellblock was modified to contain five enlarged cells, barred windows and an escape-proof ventilation system. The restored cellblock reveals the conditions as they were in 1848. Although functional by design, the Guard House’s hip roof with central chimney, corner stone quoins, slender Neoclassical columns and overall proportions demonstrate the British Military’s combined sensibilities of efficiency and elegance.