When the game says "spawning next wave" you can still use magic; you don't have to wait for the wave to actually start. This is useful for doing things like stunning pesky towers that are right by an entrance.
Great game if not for the lag. The only good thing about the lag is that it actually stops everything, so at least you aren't dead when it decides to start working again...
Don't even buy anything other than the last tier vehicles since they don't have enough health (buy up a lower one while unlocking the last vehicle). Put the repair one up front. Pretty much it.
Tip: for certain missions, like #19, you are better off not destroying the pirate ships too soon. They spawn enemies, and allow the Hellfire tower to target them and also hit the submerged divers or cloaked ninjas. Plus, you get more money by killing those guys than the one shot bonus from sinking the ship.
21 is like cheating. They go straight for the nearest frozen ball, as opposed to using the normal bounce methods. Kind of annoying, but it is the last puzzle so I guess it should be hard.
Well, the game being as old as it is had the controls glitch out on me so I was stuck in the bottom left corner...which turns out that if you stay there it makes the 50 levels with no death is pretty easy...