"Climb the Eiffel Tower" is a mission objective in Server Bridge Paris 1944
Start heading upwards. Pay some mind to the Double Jump that is available while Arno is scampering upwards along the outside of the Eiffel Tower. Remember to stay out of the search lights as this will stop you from getting shot by Snipers. That can cause Arno to lose his grip. Beyond a particular point, that is a death sentence.
When you make it to the first landing, start over to the right. A quick cutscene will run and Arno will drop down a bit. From there just head toward the center of the Tower.