For this update, I improved the enemy AI. Let's see it in action:

The enemies now fire back at the player. This also meant storing more information when firing a projectile, to prevent enemies from suiciding by shooting themselves.

Additionally, I made the enemies sometimes change direction instead of only moving in a straight line. This could do with a bit more work, to make it all more fluid and nice.

Lastly, I added a mechanic that, if an enemy leaves the bottom of the screen, the player takes a small amount of damage. This means the player is forced to deal with the enemies in order to survive.

Oh, and I also added a new item in the shop, a health refill.

I'm really unsure about what I should do next. Like I mentioned last time, I could either focus on polishing up a few areas by tweaking bits and pieces, or leaving things slightly more rough. The latter option would allow me to try my hand at different features to implement.

Thoughts?