Pellet Tower - These towers a
re extremely usefull in the early rounds. You can make great mazes with them. When you have fully upgraded them, they will become Sniper Towers with 400 damage and 180 range.
Squirt Tower - Very usefull, since they can hit flying and ground units. When you have done all the upgrades, you will get a Typhoon Tower with 320 damage and 90 range. They shoot very fast.
Dart Tower - This tower shoot a strong missle at a slow rate. But when they hit, they hit hard and deal splash damage. The last upgrade will give this tower 256 damage and 140 range.
Swarm Tower - This tower is a very good one to take out the flying units, although squirt towers can do that quite easy aswell. Full upgraded they do 480 damage with a range of 75.
Frost Tower - The frost tower slows the air and land units down. If you place them good, they can be usefull. The last upgrade for these towers is the Blizzard Tower.
Bash Tower - This tower can stun your enemies. Although i don't like them much since ther chance to stun is too low. Their last upgrade will give them 320 damage with a range of 40 and 20% chance of stunning.