This may have been requested already. If so, I apologize in advance. Not sure the exact military term for what I'm trying to describe, but the closest I think I come to it is suppressive fire. I would like the option of telling the towers to not just follow the first target that comes into range until it gets out of range, as it currently does, rather I would like the option of having the target spray or barrage a particular zone anytime there are targets in the zone. This would be most effective, for example, with flame towers or heavy artillery. You create a logjam with your maze and these towers would just go to work hitting a zone defined by you to be in the center of where you created the logjam. I know that you can tell the towers which unit to target, but it's too difficult to pull of with multiple towers with targets moving, dying, etc. I would just like it to be automatic. That way, maybe you don't necessarily aim to destroy these units, but you weaken them for other towers later on down the line to destroy them.
I also agree with others who note that the enemy's toughness way outstrips your liquidity ability to upgrade and build onto your defenses. I find around 80,000 points (not sure which wave), it's just a matter of time before I'm dead. The only recourse is the swinging door strategy, whichis is boring and dangerously close to cheating in my opinion (sorry, no offense intended to those who employe this tactic--it's just not compatible with me).