During the 19th century, progress in construction techniques, especially the use of metal and glass as construction materials, made a new type of building possible: the greenhouse. In 1873, architect Alphonse Balat designed for King Leopold II a complex of greenhouses which complement the castle of Laeken, built in the classical style. The complex has the appearance of a glass city set in an undulating landscape with monumental pavilions, glass cupolas and wide arcades that cross the site like covered streets. The Royal Greenhouses in Laeken - the "Ideal Glass Palace" - inspired the new Belgian architecture of the day, and their influence spread, with Art Nouveau, throughout the world. The present-day plant collection at the Greenhouses in Laeken is extremely valuable because some of the plants belonging to King Leopold II's original collections still exist and the current collections still respect the spirit that prevailed when the original collections were planted. Finally, the Royal Greenhouses still contain an enormous number of rare and valuable plants. As the tradition wants, each year in spring, the Greenhouses of Laeken are opened to the public for almost three weeks.