For this one I decided to take DAZ's challenge:
1) No Kills except Target who must be killed with the Hidden Blade
2) No Detects except upon Assassination Moment
3) No HUD
However, I modified the rules as follows:
1) Knockouts of Templars is allowed but not Royal Guards
2) No Awareness Meters
3) No Smoke Bombs
Skills:
4) Evie may use the Chameleon skill
Basically I used two knockouts instead of smoke bombs. If it looks easy, let me assure you that it was not. It took about 100 takes over three days to get everything right, in large part because the timing of the guard patrols are randomly phase-shifted. More about that later.
Approach
There are three well-known entrances to London Tower: the front door, the back door, and the rooftop access to the southwest tower. I used a lesser known fourth entrance: the rooftop access to the northeast tower. It is less guarded than the other tower but has a locked door. I stole the key in an earlier playthrough. Also, the spiral staircase here goes all the way down to the ground floor (the southwest spiral only goes down to the second floor, where Lucy waits to be assassinated). If there are other entrances besides these four, please mention them in the comments. There might be an open window or something that I missed.