Castillo de San Marcos
At one time, the Castillo was entirely surrounded by a moat. The water within the moat came from the bay through gates. In the 1840s the moat along the side of the Castillo facing the ocean was filled in. On the filled-in moat American soldiers placed cannons that could fire hot cannonballs at attacking ships. The moat was shallow and it could be drained at low tide. After draining, it was used to house livestock when the town was under attack.