Kinda good..
I won't care about the pixel art,as they are drawn nicely enough.
The hard part about this game is keeping units alive and gun upgrades.
About the gun upgrades.
You should make the player to be able to keep the upgrade.Because the next upgrade could be a crappy weapon with long reload time.
Then that snake control that kills back units very easily.
You should have let players to choose from snake control or just let them stay in a group.
And also when the front unit touches the screen,the back units stop.
You could let the back units continue moving when the front unit is this facing the screen.Having the units to automactically move behind the front unit helps too.
Overall its quite a good shooter.