Ninja Rush is a sidescrolling shooter in a classic mode. By "classic" I mean reminiscent of the early arcade, or the NES era.
Scrolling is forced; your ninja runs headlong forward, though you can position left and right against the scrolling background, mostly to evade enemies, and of course can jump. CTRL throws shuriken. A surprisingly large variety of enemy aliens, craft, and robots attack you; as in a shmup, each has its own attack pattern, and winning means learning those patterns and the appropriate counters.
There are three levels, and at the end of each is a boss; not a huge game, in other words, but perhaps that's good in part, since it's over before you become bored with the limited gameplay palette. It's fast, it's not easy, and it has a nice retro feel. Worth playing, certainly, if the style is of interest.