This dungeon is a part of the main quest, so you have probably done it. Jump over the wall and get to the temple.  Get close to the shining skull and activate it. 
Your task is to pass through the catacombs of Santa Maria Novella. In the first room you must use the lever located on the right.  Use beams to jump over the broken stairs. 
On the other side (at the top) you will find another lever.  Activate it and turn yourself - you have to jump through the corridor in the middle pillar and then turn right. 
Now, again, you have to use beams.  At the top, there is the next mechanism.  Downstairs the door will open - go there.
You'll see few soldiers there. Climb using some damaged columns and wall.  Kill the first opponent who is near the stairs. 
Run up and kill another enemy.  Now you have to get to the other side of the room to the next mechanism - again you will have to use beams. 
Activate the mechanism.  It will appear that there is one more. You have to go up to get to it. 
To get to the second mechanism you have to use a coffin with a skeleton that is hung on a rope and then use beams.  Use the switch. 
You have some soldiers downstairs - you can finish them one by one, fight them with swords or simply try to sneak near them.  There will another person near the door - he will start to run, so you have to chase him. 
Opponent will shut some gates before your face, so you must be cautious. If he will do it, simply find another way to pass.  At the end, run on rocks on the left and jump off to kill the guard. 
If you will fail - there is nothing lost. You will have to kill several soldiers in the next room.  After the fight use skull located near the portal.  Now listen the conversation.
Going deeper into the labyrinth you will find thee treasure - open the sarcophagus with the skull.  Now, exit the catacombs.