It's your typical action platformer: You're a ninja running through a world of demons, and your job is to advance while taking out various enemies. As with other side-scrollers, the controls are simple, this time consisting of only two buttons: jump and attack. The former is on the left, while the latter is on the right.
Your character is constantly moving through the world, so all you need to do is avoid obstacles (by jumping) and enemies (by attacking them). You can tap the jump button twice to leap higher, and tap and hold the attack button for an extra-powerful blow. You'll find a few power-ups along the way as well, including ranged weapons (shuriken, or throwing stars), and a fiery dragon that takes out a swath of enemies at once. See also: Group test: what's the best mobile game?
To see how far you've run or how many enemies you've killed, simply tap the pause button. Another button lets you switch between "normal" mode and "fast" mode, but the two are pretty similar. You have three lives before it's game over (though falling down a pit is instant death).
Assuming that you know and love the side-scrolling genre, Devil Ninja is a decent game--the high-res graphics are excellent, the sound effects draw you in, and the gameplay is diverse enough to keep you interested. The game can be frustrating, though, and it runs a little slow--even in the "fast" mode. Learning to time your jumps and attacks correctly can take a while, too, as the jumping mechanism isn't quite as on point as it is in some of the other platformers I've played (for example, Robot Unicorn Attack).