Despite how annoying you might find Preston Garvey, the Minutemen questline of Fallout 4 is a pretty rewarding one. Establishing those settlements all around the Commonwealth can yield some serious coin and benefits, and it adds an extra layer of survival into the game. However, not all quests are straightforward in the Minutemen questline.
After defeating a giant Mirelurk Queen in the Castle and taking the settlement, a new quest will eventually pop up titled Old Guns. This quest involves rearming the Castle and bringing it back to its former glory against any outsiders and potential Minuteman enemies. While not a difficult quest, it does involve a fairly tough enemy.
9 Wait Three Days After Taking The Castle
In order to even start this quest, you first need to take the Castle. This quest will inevitably pop up eventually if you interact enough with Preston Garvey since he's infamous for giving out more and more settlement quests. This settlement is located in a massive, old-time fort that needs to be cleared of Mirelurks and their queen.
After the fight, wait for three days and the call to return to the Castle should pop up through the Minuteman radio. Ronnie Shaw will request that the Sole Survivor returns to the Castle to discuss something with her.
8 Meet With Ronnie Shaw
Upon arriving to the Castle, you'll be greeted by a brand-new Minuteman character known as Ronnie Shaw. She's a Minuteman veteran, and although you're the acting general of the faction, respect doesn't come easy with her. Although a bit tough, she means well and needs your help rearming the Castle.
According to Ronnie, there's an armory filled with goodies and blueprints for artillery behind sealed doors. However, since the doors won't budge the only way in is through the tunnels.
7 Find A Way To Enter The Tunnels
There's only one way inside the underground tunnels of the Castle. Just walk around with Ronnie Shaw and eventually she'll point out an entryway that's completely overflowing with rubble and dirt. She'll ask you to clear the area so you can access the tunnels underneath.
To clear the spot, just enter the workshop mode. Hover over the rubble in front of your and scrap the trash into materials. They'll come in handy later on when you need to build some objects for the Castle. Now you should be able to freely enter the tunnels.
6 Clear The Tunnels
The tunnels aren't a particularly difficult area to traverse. It's a short underground area with mostly trash and a bit of loot here and there. Watch out for turrets as you walk inside, though they should be very straightforward to deal with. There are also a bunch of mines around, so keep an eye out for them.
Some radiation will also be present, but it shouldn't be too strong. Still, if you're worried, having power armor on might be a good idea given the boss fight you're about to enter as you walk further in.
5 Dealing With Sarge
The tunnels are guarded by Sarge, a former defense bot which is still active. It's a typical sentry bot, so the key here to avoid taking damage head on or allowing it to get close for melee attacks. Power armor will be a huge benefit here, and energy weapons in general will do a quicker job against sentry bots.
After defeating him, make sure to take note of the former general's body on the ground next to a terminal. Grab his uniform to get one of the best unique outfits in the game, which is the Minuteman general's outfit. Then, head on up to the armory with Ronnie.
4 Build An Artillery Piece
There's a whole bunch of loot in the armory, so make sure to grab what you can carry. Most importantly, there are schematics for an artillery piece which Ronnie wants you to build to increase the Castle's defenses.
Since the Castle is a terrible place to find any junk and components, you might have to leave and come back with the right items. In order to build a single artillery piece, you'll need the following items: 4 wood, 4 concrete, 4 oil, 4 screws, 5 springs, 6 gears and 16 steel. Make sure to also grab the smoke flares needed for the artillery test.
3 Test The Artillery
After the artillery is built and placed anywhere you like, it's time to test it out. Note that someone has to be assigned to the artillery before it can be used. Make sure the radio transmitter is powered up for a test and then head on over to the target area to begin the process. It's optional, but you can tune your Pip-Boy's radio to Radio Freedom.
Use a smoke flare and throw it on the target area. The artillery will then fire, and the test should be completed with that. In the future, these smoke grenades can be used to call fire on specific locations, which comes in handy when the Institute or Brotherhood of Steel decides to attack.
2 Finishing The Quest
In order to actually finish the questline itself, you need to head on over to Preston Garvey and talk to him. He'll reward you with 300 experience points, and then introduce you to the next quest which is titled With Our Powers Combined.
Ronnie Shaw will now also stay at the Castle, and she can actually be interacted with like one would with a merchant. She sells a few interesting items which might be worth going back for depending on your character build.
1 Additional Loot
During this quest there are a few important pieces of loot that might be easy to miss, so make sure to go back and pick them up. First of all, if you haven't already, grab the magazine next to the radio transmitter on the table. In the tunnels, grab the fusion core from the generator before Sarge's hallway.
In the armory, on one of the top shelves that require jumping to get to, there's a rare Nuka Grenade for any explosive type character builds. Finally, make sure to speak with Ronnie Shaw to get your hands on the unique The Last Minute gauss rifle.