The goal of the game is to get through the stage without dying. You have to kill the enemies before they kill you, while also going through obstacles. I think it's an interesting concept and a decent way to make it as close as possible to the non-mobile version.
I think the graphics could be better, but it doesn't bother me much. Definitely a good game to pass time for me.
The biggest down side is: if you stay as f2p without buying the premium version, you can't save at checkpoints. If you die, you start over, unfortunately.