It's OK if you know how to game it...
First thing - the armor reducing purple tower is indispensable in the mid game. Get it first. You can get your enemies to -10 armor (permanently) which means something that would normally do 1 damage now does 11 damage. Plus it hits both air and ground. Put a ton of purple towers at the beginning so all incoming enemies get to -10 armor or near it, and have a lot of level 1 towers in the rest of the course. Blue and red are good for this - blue because they hit air and have good fire rate, red because they have the best fire rate. With a bunch of blue towers plus the armor reducing towers you don't really need air towers.
The level 3 sniper tower is excellent and can be your main attack in the late game. Make sure to upgrade from level 1 straight to level 3 because level 2 is not worth it.
The level 3 shockwave towers (the second green tower) are the best tower against ground, even better than the sniper tower except against bosses. If they are positioned in the center of the map with the effect of armor reducing and perhaps a damage boost tower, they can deal about 60 damage to EACH ground enemy on the map, per hit. If there are 10 ground enemies... that's 600 total damage per hit, and they only cost half as much as a sniper tower. Your best weapon against an oncoming ground horde.
Bugs - on easy mode I finished every level without losing any lives (in fact upgrading my lives to 35) but it said I had only 37 perfect levels. Also, I beat the end boss but didn't get the achievement for that. Also I got the "ritchie rich" achievement when I had only upgraded my lives by 15 (from 20 to 35), and the achievement says it should be for buying 20 lives.