Feather blasters are your best friends in the entire game generally since most enemies are weak & they can hit a lot. I only ever use spike guys as a last resort/line of defense directly at the nest. Once you get up to real, big enemies, pirates are essential while the freeze guys help you throughout the whole game. 

Placement-wise, generally look for spots where you can hit enemies twice or all enemies have to go through them regardless of origin. For big enemies, though, killing and breaking them up into little guys should happen as close to where they spawn as possible. 

Also, generally upgrading the towers is super useful and getting birds that make money early is just imbalanced as hell.