I have saved the kingdom! Final score : 17015. I think it took me something like 15 tries.
The game is fun. It looks, sounds and feels like a NES game with the appropriate movements and speeds, I like the gameplay loop, the different characters you can buy have cool effects and the difficulty was perfect for my taste which made it very enjoyable to me.
Sometimes I had no sound anymore and sometimes the characters I bought all disappeared when entering a level, note that it's possible that it never happened when I hadn't already lost any princess (based on my experience it was linked with losing a princess although it doesn't always happen) so it might have happened only when I would have lost anyway. Finally I think you can only switch the second character of a princess?
Pretty cool overall!