Full Synchronisation Requirement - Do not be detected.
This is a relatively short Memory, but can prove tricky to do with full synchronisation. The easiest route to take is to go to the south where there are fewer guards, and then head over to the western side of the courtyard. Find a wall you can scale and wait until the patrolling guards are looking away from you to climb it.
Alternatively, liberal use of bombs, poison and your Assassin Signals can make clearing the guards closest to the objective relatively easy to do without being detected, just be careful to avoid getting engaged in combat.
Once you reach the tower, interact with the hatch and watch the cutscene.