Yay, I just beat Oniken.
It's 99% pure NES (an Eagle Eye will catch a cheat or two in the graphics (and maybe the music)).
Challenge was good, yet not "cheap." There's a spot or two where projectile spam can be irritating but even in those instances, it was because I rushed in somewheres (and you can sword away almost all projectiles anyway).
There's some extra stuff that opens up once you beat it, I'll get to that soon.
Speed attack achievements tho. >_<
I went in thinking it was going to be a Contra clone, but the reality is that it's more akin to some of the under the radar NES Japanese action games ala Power Blade, Shatterhand and Vice: Project Doom. There's some Ninja Gaiden in there too, for people that need an easy to digest reference, although it really is closer to the other three I mentioned.