Castillo de San Sebastián is located on two islets. The design includes nine sides, two moats and a drawbridge. The walls on the larger island are very thick to defend against attacks by sea. This is a section of crenellated curtainwall with a sentry tower on the smaller island. Strategically, the parapets facing the city are thin. The theory was if the fort was ever occupied by an enemy, the weaker defenses could be successfully breached by cannons fired from the city.