Answer: To defend the land from the British who were constantly attacking. Fort Marion had cannons along the gun deck on top that could shoot over a mile. They built the fort at the water's edge a little way away from where the water entrance to St. Augustine was, so they could easily defeat any British who tried to invade and take over St. Augustine. The Spanish built a fort around St. Augustine because, to defend the land from the British who were constantly attacking.
