My brother and I played through this and got part way through the second run before we died completely. Is there a limit on the number of play throughs, where meaningful changes will occur to the sprites and the game play? Or is there a final 'win' that you can attain?
Once you've beaten Round 5, the game will loop back to Round 1... but on the next higher difficulty. This means that if you beat the game on Normal, you'll have to beat it again on Hard, and then AGAIN on Hardest. On the flip side, you only have to beat the game once if you're good enough to finish Hardest.