Defend the Castle

Defend the Castle is a quest in Fallout 4.

Quick walkthrough
Defend the Castle Objective: Go to the Castle and defend it after you build the defenses.

The Institute synths are planning to attack the Castle. Keep in mind that you need eight Settlements for this quest to begin. You can listen to Radio Freedom for information related to the attack. Defend the Castle and speak with Ronnie and Preston afterwards.

After "With our powers combined," the Brotherhood of Steel will attack The Castle. Radio Freedom will call upon the Minutemen to defend and give information related to the attack.

Sometimes, the Castle can also be attacked by Raiders. Radio Freedom will call upon the Minutemen and give info related to the attack.

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 1 vertibird and its occupants which put together, consists of a total of 10 Vertibirds and about 30 BoS soldiers. The most efficient way to complete this quest is to use Missle Turrets. The turrets will make short work of the Vertibirds and its occupants. Most of the men inside the ship will not be lootable with the exception of ragdolls falling out. So if you are looking for loot, this method is not recommended.

Institute
Waves of Synths and Coursers will spawn at various locations. Building walls beforehand to block the holes in the wall will prevent most enemies from entering and prevent the player being surrounded.

Raiders
Waves of Raiders and Attack Dogs will attack from either the front side of the back. Again, building walls beforehand to block the holes will make things easier.