It is always good to know more about the history of a place and she seems pretty knowledgeable. As I understand it, it's a free tour and surely you have to tip at the end, I think it is fair; however, our tour guide told us that 15 euros is what they charge as they have to pay 3.5 euros for each of us and if any of us was unable to pay those 15 euros we can tell her and see if we can do the tour or not, so I do not see the point of merchandising it as a free tour. I think 15 euros is a bit high of a price specially that she did not even give us earphones so at times it was difficult to hear. I did one free tour in Barcelona and the lady told us that people usually give 10 euros and to me it seems rational, she had a microphone and earphones for everyone, this is just my two cents of a feedback. Other than that, it was a cool experience, Thank you.