Victor Horta was among the most influential architects of the early Art Nouveau, and his Hôtel Tassel (1892–1893) is one of the style's landmarks. Horta's architectural training was as an assistant to Alphonse Balat, architect to Leopold II of Belgium, constructing the monumental iron and glass Greenhouses of Laeken. In 1892–1893, he put this experience to a very different use. He designed the residence of a prominent Belgian chemist, Émile Tassel, on a very narrow and deep site. The central element of the house was the stairway, not enclosed by walls, but open, decorated with a curling wrought-iron railing, and placed beneath a high skylight. The floors were supported by slender iron columns like the trunks of trees. The mosaic floors and walls were decorated with delicate arabesques in floral and vegetal forms, which became the most popular signature of the style. In a short period, Horta built three more town houses, all with open interiors, and all with skylights for maximum interior light: the Hôtel Solvay, the Hôtel van Eetvelde, and the Maison & Atelier Horta. All four are now part of a UNESCO World Heritage Site.