Some people were having trouble with this, so I figured I'd make a quick video demonstrating a possible strategy to beat it. 5x Speed the entire game, no power ups used, and no lives lost.
In all honesty the first two towers I put down are random and fairly pointless, you can most likely just start off with the castle spam and do fine, if not better. Regardless, if you just copy what I do exactly you should beat it easily.
This is NOT a high score video, and I don't even bother to attempt to go for the bonuses. And no, my mouse cursor doesn't look like that normally, just messed up in recording.