Fallout 4 created by Bethesda Game Studios.
Images used for educational purposes only.

Prerequisites: Complete Institutionalized and Form the Ranks, or complete The Nuclear Option and By Our Powers Combined
Trigger: Varies; see below
Reward: 400 XP

Defend the Castle is a rather tricksy quest in that it can come about in several different ways, and will have you facing one of two different opponents, depending on how you got here. If you betrayed the Institute you’ll wind up facing them after Banished from the institute; if, however, you complete The Nuclear Option and are on bad terms with the Brotherhood of Steel, as well as take out the Prydwen in By Our Powers Combined, you’ll face them instead.

Regardless of how you get here, Defend the Castle requires completing Form the Ranks, which entails setting up a number of Minutemen settlements across the Commonwealth. If you’ve already established enough of these settlements ( before Form the Ranks triggers, you’ll hop straight to Defend the Castle. Speak to Preston to hear word of problems with the Castle, then speak to Ronnie to begin the assault.


If you’ve gone the Minutemen route then you probably already have a fairly substantial bead on how settlements work, and hopefully you’ve put effort into loading down the Castle with defences. Here are some further general tips that may help:
  • Load the Castle down with all the scrap you can find. Anything that contributes to repair Turrets and Generators - Steel, Rubber, Oil, Gears, Circuitry, Screws, Nuclear Material, Aluminum, Glass, Fiber Optics, Copper, and Ceramic - will be needed to repair Turrets as they go down.
  • Speaking of which, build Turrets. Build lots of Turrets. The standard variety is cheap, but they go down with a fair minimum of damage, so you may want to consider upgrading your Gun Nut perk and buying the more advanced models as well.
  • Have Wood on hand so you can build walls. This is especially important for the Institute attack.
  • Load the Castle down with as many Settlers as you can spare. Every one of them can help repel the attackers, and even if they go down fast, they’ll get up and keep helping after a short rest. Outfitting them with better weapons and armour will keep them in the fight longer.
  • Bring in every freaking companion you’ve got. All of the companions that are still friendly with you, anyway. They will happily help fight off the aggressors, and are usually better at it than your normal Settlers.
  • Load yourself up with ammo and healing items. This will be a long fight. Power Armor is highly recommended, if only to improve your defences. Sneaking isn't really a factor in this battle.

Fighting the Institute

Battling the Institute is quite painful, and easily the more difficult of the two battles. The Institute approaches the Castle from multiple directions, forcing you to split your attention between the Castle’s walls. In the later phases of the battle they’ll also start teleporting Synths directly into the Castle’s walls, and the quality of the Synths gets better and better as you go along, to the point that Coursers become standard enemies. Ow.

Key here is blocking off the side entrances of the Castle with walls. The Synths will shoot the crap out of anything that shoots back, but they seem to completely ignore walls you build and will look for other ways in. Concentrate your forces on the front entrance (the break in the wall in the northwest that faces Boston) and on the rear door in the south, blockading everywhere else. This will force the Synths into two choke points and allow your focused Turrets to chew them to crap. Keeping your Turrets within the walls will also allow them to batter the Synths that appear on the grounds when they start teleporting, as well.

After killing enough Synths you’ll declare victory. Be warned that this does not necessarily mean they’re all dead. You’ll probably have to hunt inside the walls a bit to ensure that you’ve killed ‘em all.

Fighting the Brotherhood of Steel in Fallout 4.
This version of Defend the Castle is so freaking epic.
Battling the Brotherhood of Steel

The face-off against the Brotherhood is a bit easier, though that doesn’t necessarily mean it’s easy. The Brotherhood assaults the Castle by bypassing the front door altogether and trying to drop soldiers onto your head via their Vertibirds. The waves of soldiers starts out fairly light, but gets progressively more painful as you go along, quickly resulting in Power Armor-d Knights dropping out of the sky at you. Ouchie.

Nevertheless, the Brotherhood has one big disadvantage, and that’s their stupid Vertibirds. If you can waste a Vertibird before it hits the ground, which is totally do-able, you can take out most of the people inside as well, either from the explosion or from falling damage. Ring the walls of the Castle with Turrets - the longer the range, the better, which always comes with the more advanced models - and they will batter incoming Vertibirds with gunfire, bringing them down before they can properly unleash their troop complements. Kill any Power Armor dudes who survive the fall as quickly as you can. Keep the Turrets away from the middle of the Castle, as the falling Vertibirds will usually damage them and stall your defences.

Again, you may not have killed every troop once victory has been declared in this case, but it's easier to track down the Brotherhood dudes. They're in Power Armor, after all. Kinda stands out.

What happens next depends on who you fought. If you were facing the Institute then the next quest to pop up is The Nuclear Option, the ‘final’ quest of the game. If you’re fighting the Brotherhood, by contrast, you’ll have cleared the Brotherhood of Steel out of the Commonwealth completely, and good on you for it, mate.

It’s important to note that you can get Defend the Castle twice in a single game of Fallout 4. That said, you’ll only officially get it once, during the first battle you fight, and won’t get experience for winning a second time.