* Placement is everything. Use nothing but mix towers where they can reach
two paths at once, use the others when they will only be attacking "up."
* "Powerups" are awesome if you can group 3-4 towers around it to take
advantage. Make sure they are "mix" towers so you get the bonus for both
types of enemies!
* Level 3 upgrades are not so hot, but you really don't get to use them if
you are spamming towers anyway.