Monkey Hill and the Castle of the Witch are two of the not so flattering names that locals have given to the modern building of De Gasunie. The headquarters of a wealthy gas company, the building even has an atomic bomb shelter at its base. Fake windows are meant to trick the staff into believing that they're in some natural oasis instead of locked up in a massive, solid underground bunker. Above ground a wide range of interesting architecture and design await the inquisitive traveller inside this 18-storey, 89m-high building. The two architects responsible for the building claimed that they created an ‘organic architecture that consists – like human beings – from three layers of protection: skin, clothing and building’. Don’t forget to admire the hanging gardens and the highest (60m) indoor waterfall in the Netherlands. And although we're not experts, we believe that this 45,000m2 building dating from 1994 has achieved its goal of being in close harmony with its direct surroundings.