Located behind an illusory wall at the end of an alley way in the Tower Knight Archstone area of the game is a mysterious door that wasn’t there in the original game.
Interacting with the door leads to a message that says, “It appears to be locked.” This indicates a mechanism that will open the door.
Using the game’s photo mode, reddit user Cosmic-Vagabond was able to sneak a peek at what lies on the other side. The door, apparently, leads to a terrace. On that terrace appears to be a dead knight with a small, glowing orb on it. In Demon’s Souls, orbs like this represent items.
For now though, the door remains locked.