The Castillo's St. Charles Bastion, its northeastern bastion, was the one most likely to be pointing in the direction of trouble. The British (who were always the trouble) would sail through the St. Augustine Inlet to the Matanzas River, would in short order be spotted by the alert sentry stationed in the Vigía de San Carlos (the St. Charles Watchtower) which we see atop this bastion, the alarm would be raised, and everybody would hide behind the shells of trillions of teeny clams.