Full Synchronisation Requirement - Do not create any conflict in the Imperial North District.
When the ferry ride ends, you’ll see Yusuf and his 'friends'. Intervene in the fight and defeat the three Templars as quickly as you can. After a brief cutscene, you’ll have to follow Yusuf as he leads you to the captured Assassin Den. When you reach a checkpoint guarded by Templars, Yusuf will demonstrate one of the newer tricks available to the 16th Century Assassins - Cherry Bombs. Yusuf will throw one and the guards will move to investigate, so follow him through the archway.
When you reach the next checkpoint, Yusuf will hand you Cherry Bombs of your own to try out. Equip them from the Weapon Wheel, and press and hold the Triangle/Y button to aim. Throw the bomb onto the highlighted area to get the attention of the guards, and again follow Yusuf through the archway. Keep up with him as he takes to the rooftops, regrouping with some of your follow Assassins for an assault on the captured Den.
After Yusuf and the others have cleared out some of the rooftop guards for you, proceed down the zipline and assassinate the remaining rifleman. Climb onto the roof and speak with Yusuf. Use Eagle Sense and look below you for a cape-wearing Templar. This is the Captain of the Den, and your objective is to eliminate him. The easiest way to do this and achieve 100% Synch for this Memory is to use the poison darts. Equip them and use them to take our the nearby rooftop Templar rifleman, then move to the edge of the roof. Lock on to the Captain and shoot him with the dart, then move back from the line of sight of the remaining guards and wait for the poison to take effect. When he dies, climb the tower and ignite the signal fire to automatically synchronise the viewpoint and complete the Memory.