The monorail elevator is a good navigational aid, as it's near the Fort. The note detailing Fort Defiance's location is on the table to the right ( Letter to Tommy). After opening the door, the note should be inside on the table. The key is located in the building south of the command center, inside the locker. The locked door is password-protected and will require the players to head to the storage and supplies building and open the locked room in the basement. Nobody outside the Brotherhood knew what that means - it's up to Vault 76 dwellers to figure out the mystery.
Years ago, the Brotherhood in Appalachia sent out a cryptic message saying that Defiance has fallen. It unlocks after reading the 'Fort Defiance' post in the terminal inside Abbie's Bunker.
Defiance Has Fallen is a quest in Fallout 76.
