One is frequently reminded when visiting the Castillo of the unique material from which it was constructed: Coquina stone. Locally mined, this stone comprises the shells of billions of tiny clamlets: The name means "tiny shell" in Spanish. This form of limestone is easily sculpted, and proved to be more than a match for the guns of the Royal Navy, whose cannonballs were just absorbed ineffectually into the Castillo's walls.

It is also extremely pourous, which led to all manner of decay when the NPS succumbed to public opinion and filled the Castillo's moat for 60 years, beginning in the 1930's. The Castillo was built to have a moat, but an emergency moat, only to be filled temporarily in times of danger.