Cannon seen from a few cannon rooms away. As you can see most of the cannons have been removed from the fort. Apparently most were melted down for scrap during the Second World War and the ones currently in the fort are replicas. The holes in the floor were for the carriages used to swivel the guns. Each gun was manned by a 12 man team that lived in a room behind the cannon.
Below, you can judge the size of the cannons in comparison to the two people in the picture. The photo on the right is a full side shot of the cannon.