just a warning the expansion - PixelJunk Encore - is hard... you also need 5 rainbows on the second island to get to the medium levels (thats 5 out of 6 easy levels you need rainbows).
You also need to re-get the 3 perks (running, land mines, lighting tower)
But yea PixelJunk Monsters is an awesome game and 1st challenge is beating a level - the hard part is getting a rainbow ( curse the one monster which always gets through late in the game on half the stages )