There are any number of guides for the Monuments of Paris. Many are free, some charge an entry fee and a few need reservations.
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Eiffel Tower. You can pay 14,80e to climb the stairs (first & second levels), 28e for a "Lift" (elevator) ticket to the Second Level. E-tickets are available. The Eiffel Tower opens at 9:00 and closes at 0:45 AM (last access is midnight). You must now go through a security check before entering the area around the Eiffel Tower. Plan in advance; lines will get very long in the summer. Definitely visit the website prior to your visit to see where the entrance/security gates are located.