This exterior construction is part of the defence of the arsenal which was intended to serve as a citadel. The entire exterior perimeter of the arsenal wall was surrounded by a broken line of water. It has a width of 10 metres and a depth of 5 metres. This hydraulic works has the same type of foundations as the docks (layered masonry) and was designed using mathematical calculations for the pressure of the water on the walls. Work was begun by Marín in 1752 in the park arsenal, continued from 1755 by Llobet and was completed after 1765 under the direction of Sánchez Bort in the dock arsenal. Although a quarter of this construction between the dock door and the small door has been lost, and most of the construction is buried underground, its original aspect has been recovered along a 150 metre stretch next to the park door. Two and a half metres of the steep masonry under the magistral line has been uncovered up to the level of the tide. What is more, the height of the wall has been reduced to its original height and the opposing moat wall has been restored. This construction is essential to the understanding of the defence of the historic ensemble.