Fort Pulaski took 20 years to build. The swamp had to be drained of water to have solid ground to build, 70 foot pilings were driven in to keep it firm, an eight foot deep moat was dug, and two story 7 1/2 thick walls were built. To enter, two draw bridges had to be crossed, and you had to walk under a portcullis with a cannon trained right at you.
