There are only 2 basic towers: Arrow towers and
lightning towers. Lightning towers attack air only but
they deal much much more damage against air than arrow towers.
Arrow towers can be upgraded to cannon towers. Cannons
deal splash damage but no longer attack air. There are 2
upgrades for cannons. 1 upgraded tower is stronger than many
Ice towers deal splash cold damage. They do not deal
much damage but slow down the creeps. This tower is a must
have! Poison towers deal good damage against single
units and also have slow poison. Good versus leaks and boss.
If you have all final upgrades for all types of towers then you
can build the ulimate tower: Nuclear tower. It does not
attack air but it is the best tower against ground because of
its huge damage and range.
At the beginning spend all your money for arrow towers.
Then build a town hall so you can build ice
tower. Cannon towers don't hit very well without ice towers
because the creeps are too fast. As soon as you have ice tower,
upgrade your arrow towers to cannons. 3 cannon towers is all
you need as long as you continue upgrading them. Wave 8 is the
first air wave. Build 1 lightning tower and upgrade it
to super lightning tower. Lightning towers are very
strong so you don't need many of them until wave 19. At about
wave 16 you should meet all the requirements for nuclear
tower so you can build 1 against the 2nd boss at wave 18.
After wave 19 creeps become very tough. Build 2-3 nuclear
towers and then spend all your money for anti-air or you will
die in wave 28 which is the flying boss. After wave 28 build
more nuclear towers versus the final boss.