Entry by advance online booking only (if you're lucky)!
Entry is only possible by purchasing tickets online. Many appear to do this only when realising they simply can't pay to enter (like most normal establishments). However, you can also purchase tickets for future dates, e.g the next day, which is what I did, 22 euros for 2 tickets. However, upon arrival the next day there was a wedding on in the Cathedral and access was prohibited (naturally). I complained online the next day and although replies were quick and in my own language the answer was essentially "tough"! T&Cs blah blah when the booking system allowed my payment without detailing any visiting restriction hours (which they claimed in the customer service replies). No refund, no justification. I advise you to book online as you are at the entrance to avoid paying to be told you didn't follow the rules. Why you should have to pay to enter any religious establishment is beyond me anyway - not very inclusive if you ask me. All up.. one star and that's only because there's not an option to leave NO STARS!