St. Vitus Cathedral, a gorgeous gothic structure that overlooks all of Prague, is well worth a visit. It is located on the Prague Castle grounds and you and both getting to it (by walking through the grounds) and going inside is free. Within, you will find some gorgeous stained glass windows, including one done by the famous Czech Art Nouveau painter Alfons Mucha. This gorgeous window, which stands out noticeably in style from the rest, is worth the visit alone. Beyond that, you will find many attractive chapels, as well as a massive silver tomb dedicated to St. John of Nepomuk. Overall, it's a beautiful cathedral inside and out, and not to be missed. St. Vitus's Cathedral was the highlight of my trip to Prague and the most beautiful cathedral I've ever been in (surpassing Notre Dame as my previous favorite)! I cannot stress how gorgeous it is inside; the pictures don't come close to doing it justice.
Because St. Vitus's is so beautiful, there are a lot of people wanting to visit, and when we were there the line to get in the cathedral snaked all the way along the side and to the back. I can only imagine that it is worse in the summer / peak travel season, but there is one way you can skip the line: rent an audio guide! With your audio guide you get immediate entrance to the cathedral through the exit; this easily saved us over an hour of waiting time (in very cold weather), and it also provides you with an explanation about each relic, stained glass window, and altar you see.
Double-check when renting the audio guide that this policy is still in place, but if it is - it is worth the money to save you the time waiting! Plan to spend 45 mintues to an hour in the cathedral - it is huge, with lots of people and lots to see. The tower ascent is brutal but also recommended. The view is incredible. A word of warning, however, to those visitors from countries in which (how do I put this delicately?) personal hygiene is a priority: you will be climbing nearly 300 steps in a small space with little ventilation and lots of people. people for whom personal hygiene is not a priority. Go on a hot afternoon and take your chances. It's a tricky climb in some places when crowded. Not advised for people with any physical disabilities whatsoever. I am fit and I was sore the next day.