Defend the Castle

Defend the Castle is a Minutemen main quest in Fallout 4.

Detailed walkthrough
This quest consists of several waves of attackers that spawn and attempt to take over the Castle. Before the first wave, the Sole Survivor is instructed to create defenses to protect the Castle. Afterward, a broadcast from Radio Freedom will ask all Minutemen in the area to assist the Sole Survivor at the Castle. Once this is done, the waves will spawn.

Brotherhood of Steel
Waves consist of one vertibird and its occupants which put together, consists of a total of ten vertibirds and about 30 Brotherhood of Steel soldiers. Most of the soldiers inside the ship have inventories that cannot be accessed.

Institute
Waves of synths and coursers will spawn at various locations. Like the Brotherhood, a large magnitude will also be relayed to the Castle, most often at the southern entrance.

Raiders
Waves of raiders and attack dogs will attack from either the front side or the back.

Gunners
Waves of Gunners and Assaultrons will attack from either the front side or the back.

Super mutants
Waves of super mutants will spawn with behemoths.

Mirelurks
Waves of mirelurks and a mirelurk queen will spawn.

Bugs

 * Many quests involving the Railroad, such as the weathervane, and Randolph quests will automatically fail if not completed beforehand.
 * It is a possibility that Ronnie Shaw's proper dialogue will not appear to end the quest; the only options that will appear are trading and merchant dialogue, leaving the quest unbeatable. There is no confirmed fix for this bug. However, it is possible to instantly end the quest by using the console command:
 * It is also possible that during the defend an enemy gets stuck, therefore the percentage of the objective won't move, delaying the next wave, because it never resolves. This can be fixed with a console command. Try  to jump the end of Defend the Castle (%) objective.
 * Sometimes, enemy waves will not move upon spawning or will move in the wrong direction in an attempt to fight other enemies in the area. If the quest appears to be stuck, try looking for the enemy wave nearby, and then drawing them back to the castle. Synths can also teleport into the player character-made structures.
 * Sometimes there is a bug where enemy waves will not move upon spawning or will move in the wrong direction in an attempt to fight other enemies in the area. If the quest appears to be stuck, try looking for the enemy wave nearby, and then drawing them back to the Castle. Synths can also teleport into the player character-made structures, so make sure they are not hidden in or among any place that may appear inaccessible. If the synths cannot be found, continuing the Brotherhood quests through Ad Victoriam where one must "defend Liberty Prime," synths teleport into the fight, including the synths that would have been attacking the Castle, providing the opportunity to kill them. This will also complete the "defending the Castle" mission if it was started, and not completed prior to the Brotherhood Ad Victoriam quest.
 * It is possible to glitch the quest during the synth attack in such a way that all guns, energy weapons, and explosives including Minutemen artillery will cease dealing damage. From that point on, other characters and the player character may fire or throw grenades at each other to no effect, and only bashing or other melee attacks will deal damage.
 * This quest was meant to also be a repeatable/radiant quest. However, much like with Defend the Artillery quest, looping errors prevented this from occurring.
 * This seems to have been fixed with official patch 1.3.

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