When I started working on this game, I quickly noticed the Turret behavior wouldn't be enough, because it can only target the nearest enemy or the first in range, among other things.
I started working on a custom turret system, based on events. But there were some problems, because I was having trouble with the turrets picking enemies correctly.
Because of that I asked on the forums that they added something from CC that we didn't have in C2, which is picking objects by higher/lower variable, and pick by evaluate:
<img src="http://i.imgur.com/wAL50am.png" border="0" />
To my surprise that was added right in the next beta version (big cheers to scirra!) And worked perfectly with what I had in mind
One of my goals with this game is to add more strategy to a TD.
A way I hope to achieve that, is to give more control to the player to target specific types of enemies with special towers.
So I got the most basic targeting options, like "nearest to base", "higher hp", "lowest hp" and "random". And some which are more specific, such as "Enemies on/not on fire", "Fast/Slow enemies", etc, which are more useful against enemies with specific weaknesses
<img src="http://i.imgur.com/1wDhMJi.png" border="0" />