Riverbank treasure can be available after the Moreau Boss area as you need a crank to get across the drawbridge.
How to get to Riverbank Treasure House
Head to the western exit gate out of the Altar area and go toward the Lone Road.
Take the first right down the stairs and use the crank to lower the drawbridge and cross it.
Ride the boat in the north direction to reach another dock and enter the Riverbank Treasure House on the right.
Just go straight through the doors until you find a room with three doors and a burning brazier in the middle.
How to Solve the Riverbank Treasure House Puzzle
Left Door: Shoot the brazier so that it swings toward the left torch and sets it on fire. This will make the left door open. You can collect the treasure from the left room.
Right Door: Shoot the brazier so that it swings toward the right torch and sets it on fire. This will make the right door open.
A Zombie will appear from the room on the right (Don’t kill it immediately). Let it come near, then you can shoot the brazier again to hit the enemy and set it on fire.
Middle Door: Lure the burning Zombie into the middle room near the third torch to set it on fire and kill it afterward.
This will open another door inside the middle room where you will get the final treasure – Golden Lady Statue