Nice, fast-paced experience. The pixel-art looks good! In the beginning I was overwhelmed (left/right/soot/shield - already too much for the Tiger ;-)) But then I spotted a certain pattern and "cheated" my way to victory ;-) But hey, defending is a serious task - if you are successful, everyone is happy, and nobody asks :-D
I once also implemented a Space Invaders clone - it is one of two examples for my programmable game console. Maybe you want to give it a try?