Visitors to Gustaf-Gründgens-Platz in the center of Düsseldorf, Germany, might initially think that the 90-foot-tall off-kilter box shrouded in precisely clipped shrubs is a living sculpture or some sort of oversized topiary. The mysterious volume at the plaza’s southern edge is, in fact, enveloped in nearly 30,000 hornbeam hedges. And, if laid end to end, they would stretch nearly five miles, making the installation Europe’s largest green facade, according to Ingenhoven Architects, the project’s designer. But underneath this leafy layer, and within a smaller adjacent triangular structure topped with a sloping lawn, are 450,000 square feet of retail and office space. The only hint of human occupancy, at least from the vantage point of the plaza, are a set of glazed portals inserted within the hedges that lead to the office tenants’ elevator lobbies.