Henry Mercer (1856-1930) built Fonthill as his home between 1908 and 1912 and designed it ''room by room, from the interior, the exterior not being considered until all the rooms had been imagined and sketched.'' The concrete castle boasts 44 rooms, 18 fireplaces and more than 200 windows of varying size and shape. The interior walls, floors and ceilings are elaborately adorned with an incredible array of Mercer's original handcrafted tiles. Tiles and prints from around the world also show the creative genius of this native Bucks County resident. A National Historic Landmark, Fonthill was recently featured on A&E's ''America's Castles.''