I've been monitoring the official Eiffel tower site for tickets for our upcoming visit. I've noticed that even for dates very far out (for example, today's latest booking is August 14) there is a significant gap where availability of tickets are concerned. From 17:30 to 20:00 there are no spots offered for reserve. And it always seems to be the case.
Is the 17:30-20:00 time slot pre-reserved for tour groups?