Stop a monster from falling off edge of platform

What's the trick to getting a monster to patrol a platform without falling off the edge? Is a tile type to change monster state the best method? If so, how do I make it so that only the monster changes state and not the player?

If you are using the 4.1.5 you should have "monster barrier" block (it's solid for monsters and null/walkable for others objects).
If an earlier version or NESmaker, you can try using that one: http://nesmakers.com/viewtopic.php?f=3&t=592

Wow. Thank you Dale. I don't know how I missed that.