Scam. Tour guide informed us there was no option to skip the line even though that's what the ticket was for. On that hot summer's day the line was enormous. I paid extra for access to the Towers and the guide pulled out a crumpled ticket out of his pocket with yesterday's date on it. I noticed and he quickly exchanged. Felt very dodgy. That ticket did not grant entry to the Towers but rather to the building where I paid extra EUR 10 for access to the Towers. Guide possessed less knowledge of Notre Dame than Wikipedia and kept referring to Eugène Viollet-le-Duc as that guy. Did not catch guide's name but he was a native Spanish speaker.
I have been to Paris 15 times and this was the first time I have done a tour for Notre Dame - I highly recommend it. Especially for the skip the line to the towers! It is NOT a skip the line to get into Notre Dame - no one skips that line! The guide, so sorry I didn't get his name, was super!
Skip-the-line does not mean skip the line to the catherderal. You will wait in the long, long line for the cathederal. What it does mean is that you will skip the line for the tower climb which is a good thing because those times fill up quickly. I don't understand why Viator is not completely transparent about this. The guide was great, It was worth it. Viator just needs to be transparent about what skip the line means.