Thimphu, Bhutan's capital, in addition to being the government seat, is known for its Buddhist sites. The massive Tashichho Dzong is a fortified monastery and government palace with gold-leaf roofs. The Memorial Chorten, a whitewashed structure with a gold spire, is a revered Buddhist shrine dedicated to Bhutan's third king, Jigme Dorji Wangchuck. Paro is the site of the country's only international airport and is also known for the many sacred sites in the area. North of town, the Taktsang Palphug (Tiger's Nest) monastery clings to cliffs above the forested Paro Valley. Northwest of here are the remains of a defensive fortress, Drukgyel Dzong, dating from the 17th century. Punakha is known for the Punakha Dzong, a 17th-century fortress at the juncture of the Pho and Mo Chhu rivers. In the surrounding Punakha Valley, temples include the fertility-focused Chimi Lhakhang and the hilltop Khamsum Yulley Namgyal Chorten, which has river and mountain views. The Phobjikha Valley, a world heritage site, is a vast U-shaped glacial valley, where the graceful black-necked cranes in Bhutan from the Tibetan Plateau visit the valley during the winter season to roost.