How to Get the SM Armory and Cathedral Key
In Scarlet Monastery, there are two locked doors for the higher-level parts of the dungeon: Armory, and Cathedral. In order to open these doors, you must complete the Library part of the dungeon and defeat the final boss Arcanist Doan, finally looting the Scarlet Key from Doan’s Strongbox, a small chest on the ground behind Doan.
This key will allow you to open the way to the Scarlet Hold on the living side of Stratholme as well. Rogues can also pick the Monastery doors at lockpicking 175 and Stratholme doors lockpicking 275 respectively.