William's Instant Kills and King Richard's Death
http://www.youtube.com/watch?v=s-rL2hsGhRM
---promoted to front page by Ian---
So I went back to find more things in King Richard's Citadel just prior to the scene in memory block four. And I'm glad I did, because I found a much easier way to get in there, as well as some interesting discoveries.
Easy Infiltration
When you reach the citadel go to the very left. It isn't necessary to be on the railing to prevent the scene from starting, but you have to be on the railing to reach the ledge when performing the wall run. Now for climbing across the ledge on the wall. You normally can't climb around the corner (most likely because of an invisible barrier), so at a certain place on the wall jump to the right and grasp the wall. You are now on the corner and can continue climbing to the scaffold. Now jump to the side and grasp the top half of the scaffold. From here take the obvious path to the roof to reach the other side.
William's Instant Kills
A few examples of story assassinating William before you should be able to.
1. Take out William's health with your fists (haven't tried using a melee weapon). As it turns out the gate of the citadel isn't solid unless you kill William with the hidden blade and hear the phrase "close the gates!". Going through the gate and activating the scene causes you to instantly assassinate William. Afterwards the knights still march through, but the scene is cancelled. The horses that were supposed to be in the scene are still there where they appeared; and the crowd is still there along with the invisible barriers.
2. Push William outside the citadel. Then perform a high profile assassination. Since you activate the scene while doing this the memory corridor scene starts.
I also reanimated William, but it had the same effect as taking his health out with fists. Also if you get far enough away William and the other prestaged characters respawn.
King Richard's Death
King Richard is prestaged along with William and the other guards (I can't believe I didn't notice him in the original video). Since King Richard is a well known historical figure it was worth mentioning that he can be killed in the game, especially when it wasn't supposed to be possible. Now you have the scene without Richard riding the horse (talking horse?).
You can also remove William from the scene, but you have to move him outside the citadel first or the gate becomes solid. Then do a low profile assassination to avoid activating the scene. The scene won't end though (I think it might have something to do with being detected). The chase music afterwards is a nice way for the video to end.
That talking horse did freak me out somewhat, but overall this was very well pulled off in my opinion. I seem to remember if Altiar touches the ground at any point on the approach to the scene then the usual cut scene will begin. I like how you did that jump at 0:43 seems like Altiar can jump quite a bit further than I imagined.
Oh, come on.
I tried to find a way in the first time I learned about the concept of such an instant kill, and I was satisfied that it was impossble. You are amazing.
Your insane! Is there any AC character who isnt untouchable?! hell you can even kill malik.
Told you so. 
How much time does it cost to setup the instant kill as compared to the superfast blitz+scene, because I think that may just be worth it (especially since I can sprint back to the bureau).
Also, I'm thinking you might be able to activate ghost mode with those railings, or you'd have to use the stalls to the left (the ones you use in the original SFB) to get on the railing and perform it from there. Ideas aplenty, but, alas, it's the time for finals now, so I'm busier with studying and stuff.
Told you so.
I've thought about doing an instant kill for some time. I even mentioned it to someone in chat, but they weren't interested. Anyway, I'm glad I found this route. The original route is too frustrating, but this has hardly any risk to it. Climbing over the fortress and instant killing is faster than watching the scene, so this will help in a speedrun. I also found a route that might lead you to the gate without starting the scene, which could save some more time.
It was fun recording all of this. I can't believe I didn't notice Richard before, you can actually see him briefly in the original video. And another shock was that the gate wasn't solid.