St. Vitus Cathedral
St. Vitus Cathedral, in the Prague Castle complex. It was begun in 1344 but was really only officially completed and consecrated in 1929. You know how it is with cathedrals: wars, lack of funds, iconoclastic uprisings, the occasional fire, all these things can set back your work. Basically, about half dates mostly to the 19th or 20th centuries, but the later architects were careful to use original design elements, so the whole church has a mostly unified look.
