Interesting game, at least one in which the difficulty curve is well adjusted.
Things start getting hardcore-y after wave 50, especially when waves of 8+ brown ships - that blink around and shoot in volleys - can quickly dispatch the player even when careful.
There's a glitch: Not buying the backward gun prevents the armored saucer from appearing in the game, this can make the game potentially easier since they can only be defeated with said gun (this turns out to be a trap - The player has to over them, if the next wave is a brown ships wave, you're dead, period, they'll appear on top of ya).
The secret medal requires some interaction with the armored saucer.