Basically the game play is close to what it is on the next gen consoles, but only in 2D, which takes away from it a bit. You'll find that there are three different types of level game play: normal, speedy, and then a stealth-type mode. The normal levels have you just complete the missions however you can, while speed modes give you a set time to finish them in. The stealth modes are the funnest though, as you have to sneak around and have a limited number of times that you can actually get caught - making it harder.
The game play overall could have been a bit better in my opinion, as there are just too many things that should be shown, or at least be a bit better. For example, there are too many times that I found myself just jumping off a building so I wouldn't get caught or killed - praying that I wouldn't die, and then find myself in some cart, on some landing, or something similar. It was odd to say the least. And, I'll tell you now that the mini-map provided to you doesn't help at all, and you're better off just going at it without using the map.
But, that being said - the game play overall is fun for a 2D platform game. You get to do most of the same things that you do in the Assassin's Creed versions on the other consoles, and the fast-paced action is enough to keep your heart going while you're racing around on your little Nintendo DS.