Building that is hiding this secret is in the south-western part of the city.  The entrance is in the tower wall on the roof. 
These dungeons differ slightly from the previous one - they are similar to the first one. At the end of the corridor, jump into the hay - you'll hear the conversation of two Templars. Kill one of them and chase the second one.  He will be closing gates but you will be able to get above him. Then kill him jumping on his back. 
If you will succeed you will pass the guards - if not, you will have to fight with them. Activate the next skull.  In the second phase there will be a mechanism that runs 4 rails. You need hang at all of them - only then door will open. 
The run is not too difficult - the route leads around the room. What is important is that in three places you need to jump on the railing or beam. When you will be at the end the door will open.  Take the stamp and exit.