Here's how to solve another tricky puzzle you'll encounter when trying to clear the Gray Garrison.
Start an assault on the Gray Garrison on the mission "City on Fire". Once you're on the ground floor, explore each room thoroughly. In one of the rooms you will find a demoness and traitor crusaders. Defeat all the enemies and then examine the six statues located along the walls. There are three human statues and three angel statues. In the hands of the statues can be a staff, a hammer, only a sword or a shield with a sword.
Next, examine the altar, on which lies a book with a prayer. This is the key to solving the puzzle. Read the prayer carefully to understand the sequence of interaction with the statues. There are both humans and angels in prayer. You also need to know who and what weapons used.
If we compare all the facts, we get the following:
This will open a secret room with weapon racks and one chest.