Along the Castillo's south curtain wall we have its main gate, protected by the Ravelin. Ravelins are superduper awesome, because they're like starfort shards, separate bastions sallying forth from the starfort to do damage to its enemies. Actually they exist to prevent an attacker from being able to fire cannon directly into a fort's main gate. What we're looking at here is the bridge from the Ravelin into the fort.