Once known as the St. Jago's Bastion or the Baluarte de Santiago in Spanish, the fortification was built by the Spanish in the mid 16th century and formed part of the southern defences of the city of Gibraltar, together with Charles V Wall, Southport Gates, Southport Ditch, and South Bastion.

[4][5] According to Dr. Darren Fa and Professor Clive Finlayson, both of the Gibraltar Museum, Flat Bastion was built by 1627.

[10] The complex of defences also included Southport Ditch which was depicted in the 1627 map of Gibraltar by Spanish engineer Luis Bravo de Acuña.

On that map which is held by the British Museum, the ditch is shown as a Fosso south of Southport Gate.

After World War I and reclamation of land, the Ragged Staff Gates were opened to vehicular traffic.

[11][13][14] Flat Bastion and South Bastion (Spanish: Baluarte de Nuestra Señora del Rosario) were elements in the defence of the Africa Gate (Puerta de Africa),[5] now Southport Gates.

[16][17] That smallest gate was the first of the three and was constructed by Italian engineer Giovanni Battista Calvi at the same time that he designed the Charles V Wall, in 1552.

The gate in the middle of the trio is ornamented with the arms of Queen Victoria and Governor of Gibraltar General Sir John Miller Adye and was built in 1883.

[4][11][20] It was constructed adjacent to the retired flank and behind the orillon of the bastion's west wall, which aided the gate's defence.

At the time the gate was opened in the Charles V Wall, in 1790, the prince was serving with the military in Gibraltar.

[23] Flat Bastion Magazine had been abandoned for years when geology enthusiast Freddie Gomez undertook its restoration.

[4][6][23] The men of the 150 (Yorkshire) Transport Regiment also spent a day performing repairs at the magazine.