I also thought the upgrades were enemy attacks until reading the comments - the game really picked up my second time through - I got to 11! It was fun hearing the music get crazier and crazier and see the chain explosions rip through the lines of enemy ships. 

enjoyed this one a lot, felt really satisfying getting a reaction going 

The chain reactions and the total chaos of the later levels are really cool! At first I thought the upgrades were enemy attacks, so I dodged them lol. Once I collected them, the game really started to shine :)

Simple and does the job. music and sfx from back in the days.

Got to level 26 :D

nice twist on a classic~