Slope terrain, which is a terrain that appears like walls but does not block line-of-sight. It blocks movement unless levitating or flying, and cannot be dug down on or into. If you somehow get onto slope terrain (such as by stopping levitation while on it), you can move off it in any unblocked direction. This would allow interesting level designs like pyramids and switchbacks. Is still a bit problematic without implementing actual height values for the terrain, since for things like a large pyramid, you would be able to see straight over the top of the pyramid and down the back side.