St. Vitus (or Katedrála svatého Víta) is the biggest and most important church in Prague, the pinnacle of the Castle complex, and one of the most knockout cathedrals in Europe. It's broodingly Gothic, with a forest of spires and a rose window to rival that of Notre Dame.
Enter by the Golden Portal to take a look at the stunning Last Judgement mosaic. Inside you'll find the final resting places of both Charles IV (who gave his name to Charles Bridge) and Saint Wenceslas. The chapel containing Wenceslas' remains is a stunner, encrusted with semi-precious stones.
The cathedral also contains the crown jewels of the Bohemian kings and an Art Nouveau window by Mucha. Climb the tower for a stunning view of the Castle District.