Instructions
Accept your first objective by going to the exclamation point.
Climb up the stack of crates straight in front of you. Kill the guard below you, and then swing across the poles until you get to the second set of beams. Cross these beams.
Jump up onto the ledge and defeat the guard. Go around the corner, taking out another guard in the process, and then jump up the barrels onto the ledge. Scramble through the opening on your right, and you'll hit a checkpoint.
Run to your right -- you'll end up running down a long pathway. Eliminate the guard on the roof to your left before he spots you and jump to this roof. Eliminate the guard beneath you, and then sneak down the path he was protecting.
Climb the ledge on your left, and then jump from there to the roof opposite you. Jump back from the roof to the nearest pole. Swing across the many poles until you get to a building that you can climb. Climb up the building and eliminate the guard posted at the top.
Drop down onto the roof beneath you. Use your assassin friends to destroy the group of guards below. Leap into the haystack. Walk up a winding staircase to the secret door -- it has a glowing white outline around it.