Assassin's Creed has many weapons and abilities that you may or may not know about or know how to use to their full potential, so maybe this will help you out. The control scheme mentioned here is that of the Xbox 360. If you have a PS3 or PC (custom controls!), please verify the controls with a control scheme.
 Eagle Vision
The allows you to see through the eyes of your ancestor, in which you play as, or first person view. You will also be able to use this at the end of the game with Desmond Miles. Different things and people glow different colors. Hiding spots glow a light blue and citizens do not' glow at all, besides a very faint blue. Other people such as guards, however, glow a bright blue when they are not alert or aware of your presence. The glow turns yellow if they see a dead body or are aware of some sort of commotion. When they glow red, they have seen you commit a crime and will attack you on sight. You also use this to mark spots on your map when on a "high spot".
 Running and Jumping
As a trained assassin, you can run faster than most people, just as you can jump higher and further than them as well. This allows you to navigate roof tops easier than your pursuers and escape faster. You can also run up walls to an extent. You can run up to two steps up a wall or building if there is nothing for him to grab.
You can climb almost anything that you could in reality. If there is something to grab, Altaïr will grab it. Since he can take up to two steps up a wall, this helps you reach other parts of a building, that is much harder for others to reach, faster. You can jump towards something you think you can grab, and if you don't think you will make it, press the "B" button to attempt to grab it.
You can dodge most attacks if you have your timing right by holding the right trigger and pressing the "A" button right before your opponent brings his sword down towards you or at you. You can only dodge one attack at a time though, so beware when facing multiple enemies.
 Grabbing and Throwing
You can grab the sides of buildings and other objects when you are falling by pressing the "B" button, the same way you drop from a ledge of an object or structure. You can also grab people and throw them into buildings, stalls, which will kill them and all of the people around the stall, and into other people. You also use the same button to counter someone when they grab you.
 Counter Attacks
You can counter any attack the same way you dodge, but by pressing the "X" button instead of the "A" button. Depending on the weapon, and person you are countering, you will either hurt the opponent or kill them. There are a number of different counter attacks for each weapon, including the hidden blade, which will always kill the enemy but will not allow you to block any attack, unless you can dodge them. You also cannot counter attacks with your fists.
 Combo Attacks
You can press the "X" button again, after you attack an opponent and your blade touches the opponents, and you will initiate one of the many combo attacks that will normally, instantly kill them. The hidden blade and your fists are the only weapons that you cannot preform this ability with.
The easiest to learn of all the skills. You just hold down the right trigger while locked onto the opponent to hold your guard up. Beware, if someone swings at your back, holding the right trigger down will not protect you from your unguarded back. If someone swings slowly at you, which is more powerful, they may knock you back, even while blocking.
 Strong Attacks
You can hold down the “X” button to initiate a slow, powerful strike. This does much more damage than the average swing but leaves you open to attack as well. You also need to time the swing so that you don't miss if they are on the ground or so that they don't block the attack. If they do block the attack, it will still knock them back some, which will open them up to a series of other attacks.
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