Quiet and original idea you got here for your game.
The gameplay quiet lacks purely because all you need to do is spam the spacebar in order to play. Maybe you could have added a cooldown to the space bar, or maybe had different types of rockets that took my presses of the space bar to defeat.
The sound effect when destroying a rocket is a nice touch but the text for losing the city should be a different colour as it blends in with the explosions making it hard to read.