I like your take on the subject- very to the point.
The graphics and sfx went for the very oldschool look, and it worked for it.
I also liked how the difficulty grew. You started off easy, and then slowly escalated- nice!
What I would like from this game is more of everything. Maybe a second kind of attack, more bad guys, maybe even a boss at the end.
Over all- not bad! Keep up the good work!