I did a lot of careful clambering and snooping to find the Batería's secret nooks and crannies, but...it doesn't have any secret nooks or crannies. What one sees is what one gets. The ditch around the front of the Batería would have been a substantial obstacle for attackers, if it weren't totally open on one side! Assumedly it wasn't open on one side when it was built. Or am I giving the Spanish too much credit? "Heh heh, no one will ever think to look on this side of the ditch, eh, Paco?"