The gameplay is absolutely great. There's a lot to do and it's all really easy to pick up. The core gameplay is simple. You need to go through a series of areas, kill 4 evil samurai captains, and finally kill the evil samurai overlord trying to conquer the land. This means a lot of hacking and slashing through the minions that stand in your way and a lot of acrobatics to get past the traps.
These core elements of the game play out well. The combat is fun and as simple as you want it to be. You can stick to simple slashing for most of the fights. You could also happily use kuji magic, potions, fancy acrobatics, bombs, caltrops, shurikens, and stealth to work against them. It really is your call. The game rewards you with easier fights and satisfying victories when you pull off a successful ambush. It is more than possible to keep it simple though. I really like it when a game does this. It just offers you that extra bit of choice.
There's also a lot of acrobatics. This is where the Assassin's Creed part comes in. The forest levels don't offer much, but there are 5 castle levels that require running and jumping. These are also surprisingly open levels. There are usually a few options to get through to the end. There's usually an option for straight combat, rooftop jumping, or running ropes.
As you explore, you'll find new ninjas. You can switch to them to use a bow and arrow, defeat large enemies, use a large spear, etc.
Finally, there's a big exploration side that you can delve into if you wanted. There are hidden statues in each area. There are also coins, animals to free, spells to gather, and flowers to find. Just about all of this is optional. That's important. If you want to just ignore most of it, you can. You won't have as much experience or as many potions, but that isn't too tough of an obstacle. I personally didn't find many of the game's secrets and finished with only a little difficulty. It's there for anyone who wants to just explore the levels at their leisure.
I'd also just like to add that the camera is perfect. I only ever had one spot where I couldn't see properly. That's pretty impressive in this type of game.
There really are no obvious faults. The gameplay changes at a fairly steady rate, but I do wish that they'd done a few new things with the enemies. They start to feel fairly repetitive toward the end. It wasn't a big deal though.