Defend the Castle

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

Detailed walkthrough
This quests consists of "waves" where a limited number of attackers will spawn and attempt to take over The Castle.

Brotherhood of Steel
Waves consists of one vertibird and its occupants which put together, consists of a total of ten vertibirds and about thirty Brotherhood of Steel soldiers. The most efficient way to complete this quest is to use missile turrets. The turrets will make short work of the vertibirds and their occupants. Most of the men inside the ship will not be lootable with the exception of ragdolls falling out.

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. Several turrets on top of the walls and maybe even on the castle grounds for emergency are recommended as the synths and coursers pose a threat, especially at higher difficulties where they are more aggressive and likely have more legendary variants.

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

Super Mutants
Waves of super mutants will spawn with behemoths.