This game had some good elements - decent graphics, music and controls, quite fun to play.
But in other respects was disappointing. Unprofessional sloppy typos on the help screen ("fleld", "compleate"). Lazy level design - in fact half the level maps were effectively blank, and even though later levels had more obstacles, there was very little added sophistication. Nor was there any sense of progression throughout the game - no extra challenges, waves got no harder, no new enemies, and on the flip side, no new upgrades.
Bearing in mind that all the hard-to-code parts of gameplay and graphics had actually been created, failure to employ those elements in a more challenging or rewarding way is really poor game design. Even just keeping some enemy types in reserve until later and giving them a speed boost or more subtly changing direction - wouldn't have had to be anything sophisticated! But nada.