nice concept, could use some improvements.
it was fun. but it had some issues that could be improved
1. too easy. making columns of ships pretty much made hard difficulty incredibly easy.
2. needed upgrades. you need an upgrade system rather than just unlocking different types of ships. or at least some combination of both those mechanics.
3. prep time. it was a little annoying that you kept giving us new types of ships but there was no point where we could analyze the stats of the various ships and decide what our plan would be. if you pause the game, mousing over the ship types doesn't make their stats popup and if you do it at the end of a level, the end-level message box covers up half the stat window.
4. color. some games are stylishly done in B&W. yours wasn't. it could use some color to add some depth.
upgrades man. it's all about the upgrades. that's what flash game players love most. they're so easily implemented too.