Monsters walk in air

My monsters walk in air, what do I do wrong? How can I limit the area where a monster can walk?

Monsters, knytts, and anything else that moves will automatically walk on air if they reach it. The only way to fix this is by putting either the invisible solid tile (Bank 16, Object 14) or a tile on layer 3 (that is, a solid object). Or, if you have Knytt Stories Advanced (a mod), then there's an object that's solid to monsters, but not Juni.

If you have any other questions, be sure to [search] first because this question has been answered many many times before.

Also, some creatures have a fixed path, and will go through walls even. (not those that walk on the ground though)

You should have a back story for that monster

So you wont have people complaining

Isolate them.
But there's no way with fishes