The Fort Constantine door can only be opened with 3 keys. These keys are obtained during the quest "You gotta shoot em in the head". Go to the Underworld and talk to Mr. Crowley to initiate this quest. Once you do, you will be asked to kill 4 people (only 3 have keys). If you want the awesome item that is located within Fort Constantine, it is recommended that you don't kill anyone. Just force them to give you their key as "proof" of their death. The three people who have keys are Dave in Dave's Republic, Dukov in Dukov's Place, and Ted Strayer in Rivet City. Once you have the three keys, go to Fort Constantine and open the door!