Distortion2 managed to unlock the door by collecting 30 Ceramic Coins over the course of a number of New Game+ playthroughs. The Coins are found in Fractured Mode when world tendancies are either pure white or pure black.
The reward for unlocking the door? A fancy set of Penetrator’s Armor. Fashion forward AND safe, are we right?
We solved the mystery of the hidden door! Thanks for making such a great secret to get all the community involved @bluepointgames Was awesome looking for the coins in SOTC Remake and again in Demon's Souls Remake :D pic.twitter.com/gNpozqEztJ
— Distortion2 (@Distortion_2) November 19, 2020
This new solution mirrors one found in Bluepoint’s Shadow of the Colossus remake.