Pretty excellent stuff, although it is a bit generic at this point. I didn't discover true ninjutsu because I didn't want to circle the map again.
The most frustrating thing to me was landing on or jumping into enemies because they were off screen. I know this is a gameboy thing but I feel there could be some improved design to make it happen way less. The map is enormous which isn't bad per se, but a less determined player would be demotivated by the amount of backtracking. This is compounded by the upgrades and mechanics themselves lacking novelty.