It's kinda strange. I found the key pass on the very first screen and moved on to the next one on the right. When I first enter that screen, it' not possible to walk "into" the southern wall so I'm partly hidden. But when leaving the screen and returning, it suddenly is.
Spoiler: show
Stepping to the left from that spot causes the game to crash with this note:
Spoiler: show
Also, by using diagonal walking it's possible to get on the wall.
Spoiler: show