Remember Mirkwood from The Hobbit, the forrest that was once green and beautiful but became a place of darkness and evil after Sauron cursed it? Thing is, is it possible to put enchantments (good & evil) on areas rather than just items in Skyrim? I was thinking about planting a forest at a later date then putting a good enchantment on it, more to keep evil out than to create evil like Mirkwood was. When you think about it you could create an enchantment that weakens enemies in different ways if they try to enter, making them easy pickings for you to defeat in battle.

Can't you create "zones" and then just toss a script on there to either kill/damage the types of critters you don't want entering that zone?

If creature type is undead then kill creature (psuedocode).

Is it a permanent spell like enchantments on weapons, or is it something that quickly wears off?

If you want to cover a very large area, I think it's probably easiest to make use of Locations:

http://www.creationkit.com/Location
http://www.creationkit.com/OnLocationChange_-_Actor
http://www.creationkit.com/Change_Location_Event (should also work for Actors other than the Player, but you should test that first)
http://www.creationkit.com/IsInLocation_-_ObjectReference