Different, but it works quite well.
The tried and true formula of peeking around a corner, waiting for an opening, and then going in for the kill is still very much in-tact. What's changed is going between those moments. Other Tenchu games has a stealth button, that lets you crouch down to avoid being seen at a distance. In TSA, you rely on shadows, and dashing from cover to cover, such as bushes. A big part of creating cover is taking out lights, either by blowing them out, dousing them with water, or snuffing them out with a rock or shiruken. It makes for a much more puzzle based approach.
The enemies this time around are very sharp, they take extra steps to find you if spotted, and getting into a fight is almost always fatal. Enemies come in various forms, from lowly guards, to full-armor warriors. Each enemy type has their own quirks that you have to be aware of. A seasoned warrior is very cautious, they'll look over their shoulders a lot, and give you very little room to improvise. Full armor warriors are super keen to their surroundings, and can sense you comming from behind. But, no enemy is more difficult than another, you just have to change your approach.
One thing that you'll notice from the start is that you have no weapons. Rikimaru's trademark sword is MIA. But, this goes from feeling like a disability, to revealing just how much of a BAMF Rikimaru really is. Now, your character uses the enemy's own weapons against them. You don't have to take it ahead of time, just do a stealth kill as normal and Rikimaru will take their sword and cut them down in a split second. Plus, many stealth kills have multiple animations to choose from, which can also be used strategically in regards to where the enemy falls. Weapons can be picked up, which can save you if you get spotted. If you get spotted and have a sword, you'll go into a First Person fight mode, which isn't hard to use, but does require good timing. If you don't have a sword though, you'll either disappear in a puff of smoke or die. Either way you go back to the beginning of the area, not the level, the area. Each level is divided up, so dying doesn't mean having to rekill dozens of guards, and after each death, your score for that area is reset. Your best performance is the one that you get graded on.
Items are sparse this time around, so as to emphasize careful use. The grappling hook is gone, there's no need for it here. The item you'll use the most is the water spout, which douses flames from a distance, it quickly becomes your most important tool. Shirukens are multi-use here, and can kill an enemy in one hit if used properly.
The graphics and sound are excellent. Characters move very fluidly, and much more realistically than other games. The voice acting is quite good, and cut scenes are well done with a lot of good visuals. My only complaint is that in-game characters have no mouth movement, otherwise everything is tip-top.
There are a lot of new ideas in this game that would fit very well into a serious console version. Since portable games take a different approach, the difference between this game and it's console counterparts are understandable, and work in it's favor. If you're a Tenchu fan, it's a must-get. If you're new to Tenchu, it could easily win you over.