Oh wait, I think I figured it out. My problem was I was randomly hitting buttons on my keyboard. I didn't think to randomly click and drag my mouse too.
The lorry level seems to be broken a little. I got a piece of wood stuck in my tire, and couldn't go anymore. Also, couldn't seem to drive over a corpse or piece of wood. A truck that size shouldn't be stopped by a splinter.
Sometimes, the doll gets pushed offscreen before we can kill him. I was like 20 points away from getting the crossbow achievement and he just disappeared. Then exploded into pieces on the screen again. Also, sometimes he gets pushed through objects without injury. I think it tends to happen a lot with the crossbow.
Having upgrades such as improved mana/spell regen, improved gold recovery, etc would improve the balance. There is no reason why in level 7, enemies still only give 4 gold when it's going to cost 500 to upgrade a tower. This would go a long way to fixing the balance issues. The difficulty ramps up inherently due to design, which makes adjustable difficulty levels unnecessary.
So, this version recognizes that I maxed out all upgrades on the previous version, but I still have to go through all the first levels to play the new ones?
 "Heads or Tails"){kind=icon}
 "Road of the Dead 2"){kind=icon}
 "10 More Bullets"){kind=icon}
 "SkyTower"){kind=icon}
 "Pirates Of Teelonians"){kind=icon}
 "The Keeper Of 4 Elements"){kind=icon}
 "Sky Fortress"){kind=icon}
 "Cyber Chaser"){kind=icon}
 "Evolution Mini"){kind=icon}
 "Zombotron 2: Time Machine"){kind=icon}
 "Ragdoll Achievement 2"){kind=icon}
 "Car Eats Car 2 Deluxe"){kind=icon}
use you mouse, young padawan ;)