These gun emplacements are mounted on what is called a bastion. A bastion is a projecting corner of a fort shaped to allow more gun emplacements and a heavier concentration of fire.
This is all that remains of the ten sided barracks known as the Citadel, which was destroyed during the siege in 1864.
These arched rooms were to protect the gun positions but there were rarely any canon mounted here. They were mainly used for the storage of food and other supplies.