When the Duke and Duchess of Northumberland laid out plans to create the largest public gardens in all of Europe, they commissioned the TreeHouse Co. to create a gigantic tree house that would house a 120-seat restaurant, a retail shop, two classrooms, and tow private dining rooms. Opened in January, 2005, the Treehouse at Alnwick Gardens is a labyrinth of turrets, treetop walkways, and cavernous spaces. At 6,000 square feet, it’s one of the largest wooden tree houses in the world.