To access the Old Guns quest, you will first need to have completed the previous Minutemen Quest: Taking Independence. After taking back the Castle, three Commonwealth days need to pass before Old Guns becomes accessible.
After completing the Minutemen Quest: Taking Independence, the Castle is finally controlled by the Minutemen once again. If you have been fulfilling missions for Preston Garvey, he may mention that someone named Ronnie Shaw is looking for you at the Castle. Head back to the Castle and see what’s up.
When you return to the Castle, locate a woman named Ronnie Shaw. She’s a seasoned member of the Minutemen, who wants to help you in your mission to get the faction back to its former status. She also seems to be the only person who knows how to access the old Castle armory, which is a good enough reason to hear her out.
The good news is the armory appears to still be intact after all this time. The only problem is that the armory entrance is sealed shut. Ronnie mentions another way to access the armory through the northeast bastion. Follow Ronnie through the Castle corridors into the old General’s quarters, where she points out a doorway blocked by a pile of rubble.
Ronnie gives no indication of what you’re supposed to do to clear the rubble. Don’t bother trying to blow up the debris. All you need to do is bring up the Workshop interface, and scrap the rubble into concrete. Doing this will clear the path to a tunnel leading down to the armory.
Now that the rubble is cleared away, Ronnie wants to head through the tunnels to see if the armory is still accessible. Investigate the tunnels with Ronnie, and watch out for the Frag Mines scattered throughout the tunnel rooms. Destroy the defensive turret, and prepare to eliminate a massive Sentry Bot named Sarge.
Once Sarge is destroyed, wait for Ronnie to input the password into the nearby terminal. When she opens the security door, you’ll discover a former Minutemen leader. Snag his sleek Minutemen uniform, then enter the door to the West Bastion to reach the armory.
There are some hefty armaments to be found in the Castle armory, including a Fat Man, laser muskets, smoke grenades, and other heavy weaponry. Ronnie is able to open the previously sealed entrance, allowing for access to the Castle armory from the main level.
Grab the Artillery Schematic off a table inside the armory, and then speak to Ronnie. Before leaving the armory, be sure to take the artillery supplies off the table, and open a nearby ammo crate to collect the smoke grenades and flares. When you’re ready, head back to the main Castle courtyard and speak with Ronnie once more.
You’re next task is to build an Artillery Piece that will improve the Castle’s defenses. These are the materials needed to construct the Artillery Piece: Oil (4), Screws (8), Gears (11), Springs (5), Steel (16), Concrete (4), and Wood (4).
Look along the upper wall of the Castle, and scrap the old artillery units found at each corner to gather extra materials. When you’ve gathered enough materials, open the Workshop interface and access the Special item tab to build an Artillery Piece. Although you can place the Artillery Piece wherever you like, it’s a good idea to place the new pieces where the previous artillery was located.
Note that the Artillery Piece also requires one worker for it to function. Open the Workshop and select one of the nearby settlers for the job, then select the newly built artillery to assign them as a worker. A Recruitment Beacon can also be built at this location to recruit more settlers to the Castle if you wish.
Once you have assigned a worker to at least one Artillery Piece, head back to Ronnie for further instructions.
Now it’s time to test out the Castle’s new defenses. Follow Ronnie toward the outer wall of the Castle, facing the city. She explains that artillery strikes can be called in by tossing down a smoke grenade on a particular location. This is why you needed to grab the smoke flares from the ammo crate earlier.
Walk towards the ruins of an old snack shop just west of the Castle, and get into position. Ronnie will communicate with you over the radio during the test fire, so keep your Pip-boy tuned to Radio Freedom for the duration of the test if you wish.
Toss a smoke grenade at the snack shop to signal for the artillery strike, and stand back to watch the action. The artillery will hit the target location with several powerful blasts. Artillery support may come in handy in the future, so make sure to reserve a few smoke grenades just in case.
After the artillery test, talk to Preston Garvey to conclude the Old Guns quest. Artillery units can now be built at any settlement, so consider building some at settlement locations that could benefit from the additional defense support.
Continue on to the Minutemen Quest: Inside Job, or return to the Fallout 4 Walkthrough and Guide.