Looks deceptively simple and childish, but difficult once you reach mid-game. Beautiful made math game.
Edit: lv1 is simple and great, lv2 suddenly making things more complex is the most interesting, lv3 is strange - too many buildings can be ignored, lv4 is probably intended as the bonus round - sadly if you simply try to grow your numbers without clicking on the "win button", the game will lag badly to the point of being unplayable once population reaches 6 or 7 digits.