3-4 hours walking tour - this private tour concentrates on the architecture of the early 20th century 

Art Nouveau as an architecture style of 1900's is very well represented in Prague. In comparison to Brussels or Paris, there isn't one specific Art Nouveau district, because Art Nouveau is spread all over the city. That's probably the reason why The Guardian picked Prague as the top destination for Art Nouveau in Europe... Art Nouveau is represented with both organic and linear motives wherefrom Cubism and Rondocubism later evolved. And if you have never heard of these two architecture styles, it's OK, because these styles are a Czech speciality. The tour concentrates on the location of the Old Town and the New Town as there's a lot to see. 

House at the Black Madonna - Municipal House - Hotels of New Town - Main Railway station - Appartment buildings