Brotherhood of Steel's biggest field base in Unity, unless until new forces arrive on Prydwen. The building is enclosed with a tall wall and inside, there are lots of valuable items, especially ammo and meds.
On your first stay here Danse, a member of the Brotherhood of Steel, fights waves of ghouls that attack the outpost. You can help him.
Here, you can take several side quests for the Brotherhood of Steel, assigned to you by paladin Danse. Knight Rhys and scribe Haylen can additionally assign side quests to you (securing a given area, or bringing a specific item).
Later into the game, after the arrival of the Brotherhood of Steel's ship, a dozen-or-so Brotherhood soldiers are lodged here. It is a good idea to search the building once again, because several new items appear here: an issue of Guns and Bullets (in the commander's room, in the expert safe).