It's an interesting concept; you are Desmond Miles, a descendant of the Assassins and are now the captive of Abstergo, a multi-conglomerate business seeking something that your ancestor was involved with. Lucky for them they have a machine, the animus, which allows you to relive your ancestor's life. Your ancestor, Altaïr, is an assassin working to stop the Templars plans in the third crusade. The story has you slowly uncovering their conspiracy as you assassinate key participants in the plan from three cities, Damascus, Jerusalem and Acre. As you progress the plot becomes more tangled and eventually insane, the endgame's twists are particularly absurd and impossible to predict. Whilst I usually despise this form of story-telling, it works quite well in this case; the whole story revolves around mystery, more of which is uncovered in sections in the modern day with Desmond.
The first thing that most people want to know about is the open world and the free-running. After a linear intro you are sent out across the kingdom to kill your targets with each one opening up a new area to explore. The cities are huge, detailed environments that are free to clamber over to your heart's content. They make great playground for free-running but the detail and realism means that though they feel distinctly different between cities and even districts, you'll never learn them well enough to use their layout to your advantage when trying to lose guards.
The free-running is also fun if not a bit simple, you hold down a button or trigger, or both, to control your speed which is proportional to the level of attention you attract. You are after all outside of the law and if spotted will suffer the wrath of the city guards. This can create some of the most exhilarating moments the game has to offer; trying desperately to lose the guards long enough to blend into the crowd. These heart-pounding moments occur after each assassination as you try to get back to your safe house, though later in the game when you have more moves and health you can just as easily stay and fight until your pursuers are all dead.
The assassinations themselves are the most fun and varied parts, using the intel gathered to plan your approach, get in silently and securing a quick escape route, the dockyard and book burner missions are particularly good examples of this. Sadly getting to these parts is a slog, you start each mission back in your hometown, you then trek to the city, gather intel until you have enough to proceed. This becomes very repetitive and is one of the games main flaws, a situation not helped by there being only five methods of collecting the intel.
The game is at its best when you're sneaky, using the information to avoid guards as much as possible, providing a great sense of satisfaction when you pull off an assassination perfectly, and plenty of disappointment when it goes less smoothly. What adds to the sneaking being most fun is that the combat is very rigid and repetitive, often devolving into simply waiting to counter enemy attacks. The weapon selection is small but appropriate and some fancy finishers add some spice but this simply delays the boredom.
When you're done with it you probably won't want to play it again but the story is certainly worth investigating. An interesting concept that can, if you let it, be very immersive and involving though this can be tough due to repetitive, stiff gameplay.
The Final Word: Basically an original and fun concept, sort of, that is hampered by its rigid repetitive structure. Odd considering it provides an open world.