It's a shame you can't go inside this house, which is considered the city's first Art Nouveau structure. Dreamed up in 1898 by Hector Guimard, the wild combination of materials and the grimacing grillwork led neighbors to call it Castle Dérangé (Deranged). Yet the project catapulted the 27-year-old Guimard into the public eye, leading to his famous métro commission. After ogling the sea-inspired front entrance, go partway down the alley to admire the inventive treatment of the traditional Parisian courtyard, complete with a melting water fountain. Just up the road at No. 60 is the Hotel Mezzara, designed by Guimard in 1911 for textile designer Paul Mezzara. You can trace Guimard's evolution by walking to the subtler Agar complex at the end of the block. Tucked beside the stone entrance at the corner of Rue la Fontaine and Rue Gros is a tiny café-bar with an Art Nouveau glass front and furnishings.