Brutal!  The starting difficulty was way too high for me! :(   I probably died 10-15 times before I even figured out how the basic controls work.  
I do like the central concept that the tank faces and shoots in only one direction, and that direction changes after each enemy you shoot.   That's pretty clever and did keep me on my toes once I figured it out. 
My advice would be to slow the enemies way down and then slowly ramp up their speed up, speeding them up just a little bit after each enemy is shot.

This was a fun game, although quite difficult! My best score is 22 so far.

The tank changing directions randomly (or maybe there's a pattern I didn't notice?) is a challenging mechanic, and keeps the player engaged. It's a bit unforgiving in that the game ends if you let the enemy pass or collide with the enemy even once. You could consider adding a few lives to the player. Nice music!

I liked the concept, where you're stuck facing one direction and have to compensate to still hit the target, however, I think it would have been better to have the tanks drive, instead of having to drag it. Still, good job. :)