Agreed that its a bit too easy to level. And once you are fully leveled the hardest thing to do is get all five stars on the bosses before they are dead.
It is a decent game to pass some time, but I feel it could have been much better. The game is too easy, I'm not saying you should make it absurdly difficult, but some more challenge would be nicer.
Being allowed to play older levels is okay, but that you still earn the same amount of money makes it way too easy, you can just play like the first four levels or something for a while, max out stuff and be ready for the entire thing with no effort whatsoever.
You should make it so that once you achieve 5 stars on a level you can't revisit it. Revisiting lower levels with increasingly stronger power makes it too easy to level up.
Make the lower levels return less and less money.
Simple nice and too easy
hardest part for me is trying not to kill the boss too fast to get the 5 stars