Sequence 11 - All Things Come To He Who Waits
Go with Leonard. You are going behind him and we are talking all the time.  Go to the eastern district to watch the arrival of some vessel. You will see uncle Mario. 
You must follow courier and try to be at a proper distance.  First go to the wall, kill the archer and jump on the rooftops. Go further. 
Follow the enemy - pass the temple and another roofs, at some point jump down into the water.  Then wait for a moment, because the enemy will be suspicious. 
A little further hire thieves to distract the guards.  Follow the courier and climb on the building. 
Continue your mad chase. Courier will be running in the alleys  and jumping on the buildings 
Soon he will be at the proper place. Now you have to take his place. Unfortunately, the guards will not let us in, so we can either use courtesans to disrupt their attention or get there through the back yard. In the second case, jump into the canal .
Get close to the boat and go up using its windows.  On the other side go down and murder the messenger. Take his clothes. 
Sequence 11 - Play Along
Take the box with the artifact and listen to the orders.  Follow him. 
It is particularly important not to push other people that are carrying boxes  - if you do so nearby guards will be mad. This does not mean the end of the task - try to push them very gently, take the box and run to the officer. 
During the talk, you will have two opportunities to finish the guard but it is not necessary.  After the conversation you will fight with Spaniard.  In this fight it is much better to dodge and then attack. This is not a very difficult task, but few moments later it will be more interesting.
Your enemy will call on his helpers. Again, focus on dodging, not counterattacks. As a matter of fact nobody will be able to move.  You will see your friends in the action now. 
In the next battle leave Spaniard and help the others. Enemies that are busy with fighting with your allies can be killed with a blow in the back.  Eliminate all weaker men and then focus on the main Templar.  He won't have any chances.
Ezio will talk with his companions  and later he will be marked.  Finally, jump from the tower and finish the sequence.