This is a huge Roman Catholic Cathedral in Prague, and it dominates the skyline of the city. It's a must see when exploring the castle route. This is a good free activity because the first part of the cathedral is free to enter. Inside this cathedral there is the royal mausoleum, and below that in the crypt are the royal tombs, where Czech kings and queens are interred. If you want to explore the whole cathedral though a ticket needs to be bought.