This is how you get the shotgun upgrade, which is in the fortress (area 8).

As soon as you enter, climb up to the platform with the two soldiers and take them out.
Head right and kill the grenade soldier. Grapple the leftmost corner of the ceiling in the big room, swing and grapple straight up as far left on the ceiling above as you can. Use the shotgun to begin swinging again and grapple straight up to the ceiling again.
Use the shotgun once more to swing to the door.