The Hidden Blade Assassin's Creed in Depth

An assassination of William that has another enemy take down the target.

Setup

Start by infiltrating the fortress right before the assassination (link in annotation). William's health makes it hard for him to die before killing all the other guards, so you'll have to lower it beforehand. Nineteen punches is the most you can do without knocking him out. Then start the scene.

Shenanigans

If you throw King Richard right before the scene, he'll teleport into the scene after getting up, but won't move at the same pace as the horse, resulting in a floating appearance.

Confession

To successfully confess William, he has to see another guard attacking Altair without actually seeing Altair. The target can see you through the bars on the wall around him, but there are thin potions of wall that are solid. Just make sure the SSI indicator is yellow instead of red. I chose to get the nearby posted guards' attention by killing on with a knife. The weird thing is that enemies can detect you through walls when nearby guards first detect you (probably hear them exclaiming), but in this case the enemies are far enough from Altair that that doesn't happen.

Make sure to lock onto them, getting into combat mode, or they may charge at you and shove Altair down, which ruins your chance at confession. If all goes well, William will go after the guards. Now that his health is down almost all the way, you just need to wait for a guard to perform a counter, combo, or grab and throw. Since low level guards can't perform those (actually I remember seeing a low level guard doing a grab and throw before, but it probably doesn't happen much), you might as well assassinate them. Speaking of which, it's easy to assassinate guards while their backs are turned to fight William.

Also worth mentioning is while it's easy to confess William during his speech with his men, I couldn't get it to work while he was at the table afterwards.