pf2e.fyi
From Pathfinder #146: Cult of Cinders, which isn't enabled.

Upgrade Defenses

ACTION

Requirements Crafting (master), Specialty Crafting (stonemasonry), or hired stonemason; organized specialists

You bolster Citadel Altaerein's walls with metal braces, skillful engineering, and other supports. This upgrade takes 40 days of work and a DC 25 Crafting check.

Pathfinder #146: Cult of CindersOGL