St. Vitus Cathedral
This is clearly one of the places you MUST go to see if you are in Prague. It is historically significant but it is also breathtakingly beautiful. I imagine there are a lot of photos of this structure every time there is anything about Prague. It is just something you really need to see for yourself and experience for yourself. The stained glass, the architectural design, the craftsmanship. I really can't say enough good things. If you are in Prague, swing by. You will be glad you did.