Finally I can start working on the game after I've been sick. I just made some background and a cannon which turns in mouse direction. I'am thinking about upgrade system for cannon where you can buy upgrades, such as more barrels or more powerful shots. Gameplay will be in waves and will be endless. In each wave you'll be attacked by different aliens (with gun, without gun, etc.) and you'll have to watch out for flying asteroids that may be on collision curse. When you destroy big asteroid, it will split into two medium asteroids. I was playing with the idea of primary and secondary weapon, where secondary weapon will be something like special powerful weapon which you can fire once in a while. I thought I will make this post more interesting so I will show how I did rotation of cannon :).
Vector A is mouse position and vector B is position of cannon. First you subtract A from B which will give you direction from cannon to mouse.
When you have direction you can simply compute angle between X axis and computed direction (or dot product)
Well I just hope I will be able to finish it on time.