Eiffel Tower question

You can wander around the tower and just walk up to the entrance around your scheduled entry time. There is not really a line like there is if you don't have tickets.

Do get there for your time slot.. more then 15 minutes late and you lose your reservation with no refund. So if reservation is 8pm.. you have basically from 8 pm to 8/14.. personally I would not cut it too close.. but that's just me..
Also.. line up for ticket holders is not same like as for those who need tickets.. it is however easily found and well marked. Do not get in a huge line.

Well said Pat. Thanks for providing more detailed information because yes there will be 2 lines. Also the lines for the elevators to descend from the summit can be extremely long and crowded. I believe it only takes you to level 2 then you wait in more lines (again very long and crowded) to descend to the bottom. I recommend taking the stairs from 2 down to the bottom. This gives you the opportunity to see the structure of the tower which is amazing. The stairs were my favorite part of the visit.

Showing up to early in line can be a problem as well, I got to the head of the line 20 min. before my time and was told to come back. There will be a group of you with the same time as well. The lines can be marked with Green and or Blue banners. Once you go through the initial check in, metal detector and bag search, you will head to the elevators. This gets you to the second floor. Once you get out you can look for the end of the line to wait for the second elevator ride to the third level. This line usually wraps around the outer edge of the second level. Great time to get pictures of Paris. When you get close to the elevator the single line becomes two zig-zag (Disneyland/Movie Theater) type lines before you enter the elevator.
Enjoy your trip.