In this game you are in the role of the assassin Altair and your goal is to kill nine corrupt people who have relations with the Templars, all that without compromising your identity or the Brotherhood. As you try to achieve your goals, you'll notice that there is something not quite right within the organization you're working at.
Assassin's Creed is not a long game per se, but there are a couple of things that may keep you playing for a little bit longer. That is, IF you don't mind the repetition. The modus operandi of your assassination targets won't change much throughout your gameplay. You execute an investigation, collect a couple of clues of what you are supposed to find your targets, then go for it. You can murder them silently, like a real assassin, or just run out in the open, attracting everyone's attention (which, believe me, is not a smart thing to do). Don't expect an investigation in the best Batman: Arkham series style. During the entire game, your activities will be pretty much limited to pickpocketing letters containing vital informations, eavesdropping, interrogating people by beating the hell out of them or doing small favors for your Brotherhood colleagues, like assassinating small targets or running after flags within a certain amount of time. In each mission, you have six investigation missions available, three of them are mandatory. After the third investigation has been completed, you will have enough to know where your assassination target will be and the means to get there. Kill the target, then run away from the guards and report the death to your master. It's as simple as that. Get used to it, because this is basically what you will do during the entire game.
Among the extra activities, you can also help innocent citizens to get rid of the corrupt guards. There is a certain number of citizens being treated violently in each town, and every time you save them they will make you small favors, like helping you remain anonymous or hold the guards while you escape.
One of the cool things about this game is that the guards will suspect of your actions depending on your acts. If you kill a guard, the citizens will be scared and the guards will most probably come after you. After this happens, you can either fight them out in the open until your DNA bar gets re-synched or you can escape and hide somewhere until they stop looking for you. You can hide in certain points of the scenario or just blend within the citizens.
If you just want to take a stroll in the town, you can also look for the Flags. Each city has a certain amount of Flags. You don't get much for collecting them all, but it is kinda fun to look for them.
If you think that Altair's story is too boring, don't worry. During the game you will also be able to play as the bartender Desmond Miles, which is being held as a prisoner of some weird company called "Abstergo". While he does not fight or jump for the top of towers nor kills templars, he also has to deal with a couple of problems of his own.
Problems with the game? Well, besides the repetition of events and the lack of creativity with each of the target assassinations, you will occasionally deal with a somewhat faulty camera. The jump correction also bothers a little, but both of these problems don't really affect the gameplay experience.
So yeah, regardless of having to do the same thing over and over again, the game itself is actually quite fun. You have a really big world to explore. Freedom never tasted so good.