Interesting, Poet. Thanks for the tips!
Speaking of enemy movement, I'm using a source modification to the AI (based on a tutorial by someone named Matthew) in which if the enemies are losing, they retreat and hide behind walls, waiting for the player to come looking for them, then they shoot.
The tutorial also shows how to implement an "enemy blocking" object (though I haven't tried it yet) so that where it is placed, enemies cannot move over it, but the player can.
Here's the link to Matthew's tutorial for those who are interested: