Defend the Castle

Upon arriving at The Castle, Ronnie will warn you of an impending Institute attack. You will have time to prepare defenses, build turrets, and set up artillery if you completed the "Old Guns" quest. You can also acquire Power Armor at this time.

The attack will begin with Synths approaching from South Boston. Head to the north-east to engage them. Listening to Radio Freedom can provide enemy notifications. There is a car in this area that can be detonated to damage nearby enemies. Be aware that a Courser, who may use stealth, might be present.

Enemies will then attack from the east, through the "main gate". Deal with this wave.

After the "main gate" attacks, the assault will intensify. Synths will appear from both directions and be warped directly into the courtyard. They will also appear on the walls and in the castle hallways, using doorway frames for cover (press the cover button to crouch behind walls or objects).

Switch between long-range and short-range weapons as needed. Use shotguns or melee weapons for close-quarters hallway fighting and save VATS for long-range engagements to guarantee hits.

Prioritize eliminating enemies to reduce incoming fire. Be aware that powerful Coursers will attack and can be very dangerous to your Minutemen allies. Legendary enemies may also randomly appear; loot them for legendary gear.

If necessary, retreat into the castle hallways to cover and heal, allowing your AP to recover. Preston and Ronnie are essential NPCs who will provide distractions, along with reinforcements from the Minutemen.

Continue defending The Castle until the progress reaches 100%. Enemies will retreat even if not all are eliminated.

Speak to Ronnie after the enemies retreat. The quest "The Nuclear Option" will appear.

Speak to Ronnie again, then speak to Preston to officially complete "Defend the Castle". Preston will mention that Sturges has found a way into the Institute, which is covered in the next quest.