non-pyramidal structure for many centuries. Another notable mention includes the Jetavanaramaya stupa in Anuradhapura, Sri Lanka, which was built in the third century, and was similarly tall at . These were both the world's tallest or second-tallest non-pyramidal structure for over a thousand years. The tallest secular building between the collapse of the Pharos and the erection of the Washington Monument may have been the Torre del Mangia in Siena, which is tall, and was constructed in the first half of the fourteenth century, and the Torre degli Asinelli in Bologna, also Italy, built between 1109 and 1119. World's highest observation deck Timeline of development of world's highest observation deck since inauguration of Eiffel Tower. Higher observation decks have existed on mountain tops or cliffs, rather than on tall structures. The Grand Canyon Skywalk, constructed in 2007, protrudes over the west rim of the Grand Canyon and is approximately above the Colorado River, making it the highest of these types of structures. Timeline of guyed structures on land As most of the tallest structures are guyed masts, here is a timeline of world's tallest guyed masts, since the beginning of radio technology. As many large guyed masts were destroyed at the end of World War II, the dates for the years between 1945 and 1950 may be incorrect. If Wusung Radio Tower survived World War II, it was the tallest guyed structure shortly after World War II. Tallest towers Towers include observation towers, monuments and other structures not generally considered to be "habitable buildings", they are meant for "regular access by humans, but not for living in or office work, and are self-supporting or freestanding, which means no guy-wires for support", meaning it excludes from this list of continuously habitable buildings and skyscrapers as well as radio and TV masts. Bridge towers or pylons, chimneys, transmission towers, and most large statues allow human access for maintenance, but not as part of their normal operation, and are therefore not considered to be towers. The Tokyo Skytree, completed in February 2012, is , making it the tallest tower, and second-tallest freestanding structure in the world. History of tallest tower The following is a list of structures that have historically held the title as the tallest towers in the world. Tallest structures, freestanding structures, and buildings The list categories are: The structures (supported) list uses pinnacle height and includes architectural structures of any