The entrance to the tunnels is difficult to find prior to beginning the quest Old Guns. It is hidden behind rubble, and cannot be accessed unless the workshop for the Castle has been unlocked by advancing in the quest Taking Independence.
Once the workshop is unlocked, head to the room in the east bastion. Then, look to the left to find a pile of rubble which can be scrapped in workshop mode. Upon scrapping this rubble, the stairs leading down to the door to the Castle tunnels will be revealed.
The tunnels are winding but essentially linear. Hazards include mines, explosive gas, radiation, and a turret. Storage areas are encountered first, then an area with a chemistry station and a generator, followed by a barracks with usable beds. Next is a locked security door guarded by a malfunctioning, hostile sentry bot named Sarge; this door can only be unlocked by Ronnie Shaw via the nearby terminal during the quest Old Guns. Behind the door is a room with the corpse of General McGann and the exit to the armory in the west bastion.
- Ten cannonballs - On shelves in one of the storage areas early in the tunnels.
- Fusion core - In the generator.
- Two Laser muskets - One on a shelf in the room before the generator room, and one on General McGann's corpse.
- Seven full bottles of Montressor Amontillado wine and four empty ones - In a crate and scattered near General McGann's corpse.
- Minutemen general's uniform and hat - On General McGann's corpse.
The Castle tunnels only appear in Fallout 4.
Behind the scenes
Toward the end of the Castle tunnels, near General McGann's body, is a crate of Amontillado wine. Opposite the wine is a skeleton seemingly entombed in a wall. This is a reference to Edgar Allen Poe's story The Cask of Amontillado, where the protagonist's enemy is lured into an underground vault, chained and then encased by a wall. The label of the Amontillado Bottle ("Montressor, Amontillado, Bottled by: P. Edgar") is again a direct reference to the work of Poe and the protagonist, Montresor.