That's kinda hard I'd think, but nt impssible. A way you can do this is first fill the level up totally with masked tiles, and then cut out passages where you want people to go. Most of the space will then be used up with masked areas where ppl cant go, and they can only move where you made the passsages. I've made a few levels like this, and they do turn out to be bigger than the ones where you paste platforms in. It's a more complex idea of Unknown Rabbit's humorous idea

That's about all the advice I gan give, good luck!!!