I added an economy to the game. This is the ultimate mechanic needed for the demo. The main and so far the only currency the player has is stars for completing levels. Stars are awarded after winning. Each level can be completed again. But each time the difficulty of getting a new star will increase. For example, in each level to get the first star you just need to destroy all the opponents. The next time you will need to destroy everyone, spending no more than 12 bullets. Then there is a time limit. Thus, in order for the player to buy upgrades, he will need to complete increasingly difficult tests.

You can also get stars by getting achievements. For example, if you destroy 50 enemy ships in total over the entire time. This is another way to buy an upgrade if you can't complete the levels.

I also improved some screens to make them clearer. And fixed a number of bugs.

What mechanics do you think the game is still missing?