These hints belong in general to towerplacement.
All of towers have splash damage, exept shadow. Shadow tower has good range. So analize what you need in you situation , more splash damage against crowds, or more single fire against bosses or small crowds. Upgrade towers in according this thing.
Start your defense with slow towers in any map. It deal good splash damage and slow. Combine then with all another towers. So you can make very cheap and strong defense in the start.
[IMG]http://s58.radikal.ru/i162/0905/64/f9a150415cdf.jpg[/IMG] 
Don't upgrade towers more than 1 time ... first upgrade make good increase damage (or range for shadow) but further make more little and little increase.
Try to combine fire slow and earth towers and uprage some ot them to lvl 2, i think it's near the best combination and don't build fire towers near another (it will do nothing damage)
And i advice you to not build light towers, because IMHO they don't compensate itself, much better to build 4 shadow towers for example.
