With the fate of the Commonwealth almost decided, only one obstacle remains in the way of the Commonwealth Minutemen and their general: the Brotherhood of Steel. With their leadership based out of the Prydwen behemoth, a plan must now be enacted to take it down. Permanently.
To increase the Minutemen's firepower for the upcoming battle, Preston will instruct the Sole Survivor to build artillery at five other settlements outside of the Castle, and have each of them actively manned by settlers.
Upon completing this task, visit the operator for Radio Freedom at the Castle, and ask him to give the strike order. Upon sending out the order, the Castle will come under attack by Brotherhood of Steel forces, in which the Sole Survivor will need to lead the Minutemen to victory against this threat.
After the assailants have been dealt with, the Sole Survivor will watch from the Castle as the Prydwen is brought down by the condensed artillery fire, in which the surviving Brotherhood of Steel, arriving from vertibirds, will launch a final, desperate attack against the Castle, all of which must be dealt with in order to eradicate the Brotherhood of Steel threat from the Commonwealth.