This is a pretty barebone game, you might as well combine the 3 level into 1 as they pretty much identical to each other just with small addition. even with just 3 enemy type there many way to present a challenging level, repeating the same enemy pattern over and over without any thought put into it is not a good way to design a level.
There also some glitch like despite i already getting 3 star on each level the level selection is not displaying the correct star at all. the 1 stage show the correct star but the other level not.