cathedral of barcelona

The Cathedral doesn't require reservations. I don't even think you can get them, but a line does form at times. We were there during Semana Santa; although Barcelona doesn't do much special for the week, the cathedral did have extra closures for mass. They also have a sign near the entrance with their dress code (first time I've actually seen a sign showing the policy). As I recall, there are three different sight choices - admission, treasury and elevator to the roof. Sometimes, admission is free. A ticket that covers everything is about 8 or 10 euro.

Also, do you mean the medieval cathedral or the Sagrada Familia Basilica (not a cathedral)? If you meant the latter, that doesn't need reservation either.

Both the cathedral and Sagrada Familia are worth visiting...completely different in approach and in feeling.