MINUTEMEN QUEST 07 - Defend The Castle
Triggered by completing either Minutemen Quest 06 - Form Ranks or having The Institute hostile to you and talking to Preston Garvey after having 8 or more Minutemen settlements.
Note: In preparation for this quest (especially on higher difficulties), you may want to build defenses beforehand at the Castle to make things a lot easier for yourself. Or at least transport all the parts required to build defenses to the Castle to allow yourself to build everything without making multiple trips to your usual item dumping location (i.e: Sanctuary in my case).
Note: You will be fighting massive numbers of synths and coursers during this quest, so make sure you are well outfitted (bring power armour) for the job.
Objective: Travel to the Castle
After starting this quest by talking to Preston Garvey, you'll need to make your way back to the Castle. As mentioned in the note above, you may want to start building defenses before continuing with the objectives because after you reach the objective 'Build defenses' you will be on a timer as the Institute will attack a certain amount of time afterwards (in my case a minute or two afterwards, however that may be due to fast traveling to and from Sanctuary for parts).
Objective: Build defenses (0-100%)
Your defense total will have to be around 50+ to top out the percentage for the objective (I had 28 which resulted in 56%). However you don't actually need to build defenses for the attack (it will come regardless). Building turrets does make things quite a bit easier though in dealing with the waves of synths. So if you have the parts to hand, take the time to build as much as you can. Make sure the defenses face the South if they are a cone of sight type turret (like shotguns and missile turrets) as that is where the initial attack will arrive from.
Objective: Defend the Castle (0-100%)
The attacks will comprise mainly of synths with coursers becoming more frequent the longer the fight progresses. There are a finite number of oppoents however, so the percentage counter will update by 10% at a time as you kill your way through. The main initial strike will come from the South, so focus your efforts there first.
The enemies will likely spread out more and work their way inside as the fight goes on so you'll need to be aware of stray enemies flanking you. When things get too hectic you can head inside the stone hallways that make up the walls of the Castle near the armoury for extra cover. The battle is quite long, even with a good loadout and preparation so make sure to bring sufficient aid items and ammo for your guns.
Objective: Speak to Ronnie Shaw
After the battle finally ends, track down Ronnie and she'll point you to Preston who has heard some interesting news.
Objective: Talk to Preston Garvey
When you catch up with Preston, he'll inform you that the Minutemen have finally found a way to infiltrate the Institute. After the little chat you'll need to report to Sturges for more information as the quest ends and the final Minutemen quest is triggered.