"First, this is not AC3, it's sort of an expansion like Brotherhood was. This game is supposed to clear up all the questions thusfar and bring a closing to the stories of "current characters".
The game takes place in Constantinople at the beginning of the 16th century. The city is ripe with Templar activity and they fiercely battle the Ottoman. The city is said to be bigger and more populated than Rome, and will be split up into 4 large districts.
You will once again be in the shoes of Ezio, who is now in his 50s. The game starts with Ezio going to Masyaf, the home of Altair in AC1, to look for information on Altair and the Creed. Ezio discovers that Masyaf has been overrun by Templars and after doing some investigating he learns of an extremely powerful artifact hidden inside of Masyaf. To get to it, he must find 5 ancient seals in order to gain access to it, however the Templars are also going after it.
Ezio will make new friends, such as Suleiman, Yusuf Azim who is also a master assassin, and Sofia Sorto who makes Ezio question what his life would be like if he had never had to become an assassin.
The Seals that Ezio seeks will also give you a glimpse of Altair's life and you will get a chance to play as Altair during these small sessions.
Ezio will of course have an all new arsenal, but two of the main things are the new Hookblade, which will be attached to the hidden blade and allow Ezio to climb faster, grab enemies, ride on zip lines, and be 150% more badass. It's basically just a small blade with a curved end.
The other new feature is Bomb Making. You will be able to craft over 300 different types of bombs from flash and smoke bombs to explosive and even bombs that drop caltrops to stop chasers.
Eagle vision will also be much more advanced, you will be able to see where somebody, for instance a guard, has recently been or soon will be so you can lay traps in advance.
Borgia Towers will return but will be much more advanced and offer more variety. Assassins will be more in-depth and you can even get missions from them as well as lose control of a "Den" you have previously taken over.
There will be random missions, such as a merchant being robbed or a woman being assaulted that you can choose to do something about or just leave alone.
The game is using Mocam which is the same technology being used by L.A. Noire
The cinematic camera is also being changed.
Desmond is in a coma from the end of Brotherhood, and the two assassins who find him put him back into an animus, as such he is basically trapped inside his own mind. Players will play as Desmond as he tries to reconstruct his mind and wake back up. Through doing this, players will also get a look into Desmond's past life. Desmond is basically looking for a "nexus" of where him, Ezio, and Altair all come together and why they do.
Finally, multiplayer will return but they promise it will be much better. Multiplayer will now be more story-driven to allow players to get a more in-depth view of the Templars and what they stand for. There is a new gamemode that will involve mutliplayer story quests. There will be more customization and more characters as well."