Instructions
Head straight for the castle gate in front of you as the dream sequence begins. There will be many guards around, but you can ignore them. As soon as you step inside the castle, notice the glowing figure running away.
Lock onto the target and try to keep up with him.
Follow your target up the ladder to the roofs and continue chasing him onto the parapet with the castle tower at its end.
Climb to the top of the balcony railing halfway up the tower and stand beneath the jutting wooden beam.
Let go of the left analog stick and hold the "R" trigger (the "high profile" button), then tap jump. Altair will grab the beam, allowing you climb to the top of the tower, triggering a cutscene.
Perform a leap of faith off of the top of the tower to end the dream sequence.