Arno will notice that Elisa and her father are just ahead in the nearby gallery. He needs to make his way there. Perform the prompted leap of faith into the haystack below you to the South. Head on down to the ground level from there and go over to the nearby crowd. As you approach the tutorial on the usage of Eagle Vision will be prompted.
You need to use the dense crowds to sneak into the gathering. If you are caught you will just be thrown out and have to start over. Head through the center of the large group of people in the gateway. Make your way over to the. Now just wait fro the guards to have their backs to you then slip over to the next small group of people. Use them as cover then head to the coach. From there, move to the next group of people. After that carefully blend into the next crowd. Make your way down to the next red carriage. Start over to the door ahead of you.
Head on through the door and go through the next one on the left. From there just make your way to the back of the room. You will see a door that is marked to be opened. Head over to it and do just that.
You will be prompted to start climbing upward.