Kitten Squad is a free and fun twin-stick shooter game, easy and accessible.

While it can be hard at first, you will soon understand the best way to defeat each enemy and after several tries, you'll be able to finish every main quest without any struggle.
After some time the game will start to be quite repetitive as this game doesn't have many types of enemies, so after completing several runs, it will become boring and repetitive.

All in all, this game doesn't give you any challenge and after few hours it becomes boring due to repetitive enemies, but well, after all, it's free.