In a city filled with baroque palaces and numerous architectural adornments, this sprawling public-housing project in the rather bleak 3rd District is visited -- or at least seen from the window of a tour bus -- by about a million visitors annually. Completed in 1985, it was the work of self-styled "eco-architect" Friedensreich Hundertwasser. The complex, which has a facade like a gigantic black-and-white game board, is relieved with scattered splotches of red, yellow, and blue. Trees stick out at 45-degree angles from apartments among the foliage.

There are 50 apartments here, and signs warn not to go inside. However, there's a tiny gift shop (tel. 01/715-15-53) at the entrance where you can buy Hundertwasser posters and postcards, plus a coffee shop on the first floor.