Opening the menu requires pressing control, control exits fullscreen, and exiting fullscreen crashes the game (at least on the browser version). Also, there's an area (I think it was called the Ruined Cathedral?) where if you walk to the right you can't go any further. I don't know whether you're meant to come back there later, but if so it should be more obvious instead of just being able to walk slightly offscreen
Viewing post in ERICA, Knight of the Sun comments
I think I know the place you are talking about. I got confused there and the moonlight area at first. Both are linked to finishing questlines first.
I agree though. It gets very confusing when you can or can't go to the next area. It gets a bit more confusing in the town where certain doors only are able to be entered. I think there needs to be something just to indicate when you can go into an area (just something simple like an arrow above the player's head)