[zokemon]

The onwall type is based on a few things:
Tilelayers have 2 different types of "blocking" tiles and two different types of "unblocking" ones.

Each one has a foreground and a background type. I haven't looked into it much but I'm pretty sure the foreground types will override any background types (be it blocking or not) that are above it. Otherwise, the top layer is the one that take priority in choosing the actual tiletype.

Onwall wouldn't work with just one layer because that means you are saying you want to check npcs, players AND a single tilelayer? Kinda funky.