Only one part of the Eiffel Tower regularly sells out weeks ahead, and it is the top. The summit platform sits 276 meters up, more than twice the height of the second floor, and you can only reach it by lift from the second floor. When it is full, the ticket office simply stops selling summit tickets until space frees up. A pre-booked summit ticket is the only thing that guarantees you a ride.
The official summit lift ticket costs €36.70 for an adult. Below is what you get for that, how to make sure you are not turned away, and every guided tour we could verify, split into the ones that include the summit and the ones that sell it as an add-on. For second-floor and stairs tickets, see our full Eiffel Tower tickets and prices page.