The Hidden Blade Assassin's Creed in Depth

Sequence 3 Memory 2 Confession

The first target in the game, Sivert. The plan is to run through the mission unseen, which means no interaction with enemies unless necessary. Stealing the key seems to contradict this idea, but since it's an optional objective and it just happens to be guards you're taking it from, I wouldn't make a big fuss.

The two guards stop inside a building for a talk. Crouching on the stairs, you can take the key behind the guy's back. After the key is retrieved, the guards walk away (how did they know?). I've been seen a few times; it was either because I wasn't in the right place or because sometimes the guards don't always face the same directions after they've stopped, something I've attributed to getting separated by the crowds of civilians on there way to the stop location.

Another way to do this would be to blend with the crowd inside the building and take the key while the guy is passing through, but I preferred to demonstrate this way because it shows where they stop.

Ghost Run

After getting inside the cathedral from the North side, you don't have to worry about guards on the second floor. The only character you'll run into is an armed civilian who seems to serve no purpose (other guards from the second floor can't reach this area). Use eagle vision to get the target moving and get to the altar. The brute nearby won't be a problem because he's not facing the altar directly.

Hide behind the altar on the side opposite of where the target enters. Using a corner kill is stealthy enough not to get seen by him or the nearby brute. Even afterward the body doesn't get discovered (big AI fail, the guard even turns his head that way).

Now climb back up the way you came. One convenient entrance/exit is an open window on the north side.