Castillo de San Marcos
The Castillo is built of large blocks of a kind of rock called coquina (koh KEE nuh). Coquina was cut in a quarry on nearby Anastasia Island and taken by raft across Matanzas Bay to the site of the Castillo. Coquina was ideal in one way for a fort--when cannonballs hit, the walls were not broken, but only dented. This happened because coquina is very porous, or full of tiny holes. These holes, however, caused a problem. They let water pass through the walls, making many of the rooms in the Castillo damp. This was bad for soldiers, bad for food, and bad for gunpowder. To solve this problem, the outside of the fort was painted. The paint kept out much of the moisture. Being white, the paint also made the Castillo easier for ships to see.