Reminds me of the Commando Flash Games that I used to play as a kid.
Art looks great. Love the colour contrast between the foreground and background. Skyscrapers and Tank were my favourite bits. Good interpretation of the theme.
For improvements, the pacing feels a bit slow (the time taken to unlock a new weapon and how tanky enemies are). I'd suggest upping the players and the enemies damage to speed it up while maintaining the difficulty level (which was quite good).
Cool submission. Good job!