Chram Svateho Jiljí

Beautiful, historic cathedral in the heart of the Prague Castle complex. The main style is Gothic with richly decorated sidevaltars. You can visit just the back of the church for free, but must get a ticket to see the whole thing (well worth it).
The chapels behind the main altar house the bodies of many early Czech kings and there are beautiful stained glass windows. Some are old, some arevrelatively modern — including one by Mucha.
I’ve visited major cathedrals in Paris, London and Rome and have to say that I was blown away by St Vitus. It’s uniquely Czech and something you should see when in Prague. My advice would be to visit after lunch as it’s truly awful in the AM with hundreds of people from tour groups clogging the place up!