Here are few suggestions for improving of your game:
1. I would recommend you change the way how enemies shoot. It seems weird when player is on the left side of the enemy but the enemy is shooting toward right. So I would recommend implementing a simple check so that if players if left side of the enemy the enemy would actually be shooting toward left and if the player is right side of the enemy the enemy would be shooting toward right. Same goes for shooting up or down. This would make enemies a bit smarter and also increase the game difficulty a bit so you could implement this as a staring option for changing of game difficulty.
2. I would also suggest implementing the ability of enemy to go and fire a dense salvo of shots toward the player when player remains in the same line or same col for certain time. This would make destroying of enemies by firing a dense salvo of shots a bit more difficult. Right now you could risk of being hit by a few shots yourself while raging a shooting salvo at the enemy. This could also be as an starting option for changing of game difficulty.
3. Another suggestion of mine is implementing damage multiplier. When I'm watching of your video the game pace seems a bit slow. But if you add a damage multiplier with which players can chose that all shots deal more damage it could increase the game pace a bit.
4. You should definitely implement a hot seat multiplayer support. Fighting enemies with your friend in Co-OP mode would be more interesting.

That is it for now.