Castillo de San Marcos
If you were to turn around once you entered the Castillo, you would see a very sturdy wooden grate that slides to shut off this area from the drawbridge. This grate slid closed behind the drawbridge. If attackers managed to get past the drawbridge, this wooden grate, which is reinforced with iron straps, would provide another way to prevent entrance. It would give defenders a better chance against any enemy trying to enter.