We did the tower trip a couple of years ago and did book it (includes a tour of the main floor, too) before our cruise. I several times were offered and we didn't have any issues. However, during the summer I would bet that it is in high demand...do book early. Side note: Dan Brown's new book, Origin, takes place in Barcelona and in several Gaudi buildings. Read it before you come, tour the buildings and relive the novel these interesting facilities, including the cathedral. As stated, the elevator trip is one way. You must walk down narrow, rather dark stairs (a lot) with no hand rails (until the last 100 feet or so). If you are claustrophobic or have a fear of heights, this adventure might not be for you. There are narrow landings here and there as you descend where you can get great views/photos of the top of the structure which requires another 40 years to finish. A couple ahead of us had a teen-age daughter who was so scared to descend that she had to stop many times and have the support of her parents to continue. We found it unique, historic and fascinating. Gaudi's other buildings and Parc Gruell remind me of Whoville...who can turn away from Dr. Seuss??