The Royal Greenhouses of Laeken, Brussels

King Leopold II had the architect Alphonse Balat build this private city of glass in the grounds of the royal palace at Laeken, starting in 1873 and adding to it for the rest of the century. The green-painted domes seen here rise above long glazed galleries that connect the pavilions, so you can walk a long way without stepping outside. The greenhouses still belong to the royal family and open to the public for only a few weeks each spring, with tickets released online beforehand and selling quickly.