The engineer noted that one of the castle’s major weaknesses was the lack of a surrounding moat. Traditionally, the different sections have been named according to the nearest bastion, with the exception of the stretch between the Velasco and Santa Amàlia Bastions, known as the Santa Eulàlia Moat.
The records suggest that the most important tasks involved in the construction of the new moats were concentrated in the sector of the lunettes and Llengua de Serp Bastion and the front part of the castle. There is also talk of some kind of change of level in the Santa Eulàlia Moat, before Cermeño’s intervention.
The moat was excavated out of the natural substratum of the hill, to a depth of about three metres below the level of the covered way, and it varies in width depending on the situation of the adjacent bastions. It is bounded on the inside by the scarps of the bastions and the curtain walls connecting them, and on the outside by the counterscarps of the covered way, reached via stone steps. Small doors called posterns communicated the bastions and the moat. There are three of them, located on the right flanks of the Velasco, Santa Amàlia and Llengua de Serp Bastions respectively.
The purpose of the moat was none other than to improve the defences of the castle because it increased the relative height of the surrounding structures and created an enclosed, sheltered space outside the inner ward. It was also used as a perimeter drainage ditch to get rid of surplus water from the castle. In this sense, both the cistern located in the Santa Amàlia Bastion and the pond located in the outer ward, had some openings that let water in from the moat.
With the conversion of the castle into a military museum the moats were landscaped and Santa Eulàlia area was made into an archery range. Subsequently, some spaces were dedicated to the memory of those executed there in the first years after the Spanish Civil War, and to Lluís Companys.