In an interview with The Washington Post, PlayStation's external development creative director Gavin Moore revealed that there were early plans to have a more entry-level experience for series newcomers early in development.
Bluepoint Games had made this addition in the remake.
As it turned out, along with the locked door, the developers added several dozen well-hidden ceramic coins to the remake - they can only be found in mirror mode with a completely light or dark world trend. He got the credit for being the first person to open the door after he shared the following image on Twitter.
The door lies beyond an illusory wall in Boletaria world 1-3 and there's no key anywhere nearby to open it. The design on the coins is similar to that on the door.
Next, trade the coins into Sparkly The Crow in the Shrine of Storms to get a rusted key. Trying to duplicate Ceramic Coins in multiplayer by dropping them for another player and disconnecting transforms them into Broken Coins, which at this point seem useless. The rewards weren't exactly world-changing for using it: The room on the other side contains a full Penetrator armor set and a sword. In fact, Bluepoint Games tweeted an image of the verandah to the Demon's Souls streamers.