How do I prevent monsters from falling off their platforms?

Don't know if this had been asked before. I did a quick search but didn't find anything. I am helping Redherring32 with his platformer game. Like the title says if a monster is set on top of a platform it will fall down to his peril. Other than adding rails is there a way to fix this?

Here's a link to a video recording showing what I mean: (links to dropbox)

[media]https://youtu.be/kI65w4sg3A8[/media]

You would need a "monster block" tile type... here:
http://nesmakers.com/viewtopic.php?f=3&t=592