Invisible cubes - Invisible wall - Block creatures from walking somewhere

[yani]

I would like to have some sort of way to block creatures from going on a slab. Preferably with cubes.

My idea is to have an invisible cube with collision. Maybe by using a texture.

Some examples:

• When you make a "prison like" wall with 1 cube spaces but you don't want to allow creatures to walk trough them.
• When you use the black floor texture so you can make "abyss" type of slabs that aren't able to be crossed.

Maybe it would also have some use if there would be an invisible cube without collision that could be used to keep the ceiling from pulling down.