For someone who has not played Ninja Gaiden since his 2D games this was a refreshing change, to keep within the DS hardware limits the levels are Pre-rendered scenes while the characters and any Interactive objects are 3D models, to my knowledge in terms of the games viewpoint this plays like the more modern 3D Ninja Gaiden games.
The combat can be challenging to the point of unfair at times but that's just how Ninja Gaiden games play. You navigate areas, find keys,spells,pull switches, dodge traps and flip jump to climb up vast pits all while kicking demon butts and bosses along the way. Which brings us nicely to the touch screen controls, although veteran gamers in particular will be baffled by the touch screen only controls at first they have been designed in such a way that you never feel like your attacks are totally random.
You hold the pen on the part of the screen you want Ryu to move towards, tap it to shoot or hold down block to do a rolling dodge and in combat you literally slide the pen across the screen in sword like slash motions to do different attacks or slash upwards to go into a jump which has yet another new set of moves. The combat can be repetitive at times but how much that bothers you will depend on how much the combat appeals to you, personally I feel the combat alone in this game is one of the most revolutionary things to come from the DS
P.S. you may need some kind of touch screen protection with this game because it single handedly killed my generation 1 DS's touch screen, but this was more down to how old my DS was then the game itself.