The Tai was an ancient architectural structure, an elevated terrace with a flat top. In most cases be built of earth, stone and surfaced with brick, they are used as a belvedere from which to look into the distance. In fact, however, many well-known ancient Tai today are not just a bare platform but has some palatial halls built on top. A typical example is the Round City of the Beihai Park in Beijing. As a terrace five meters high, it has an area of 4, 500 square meters on its top and a main hall with side corridors.
The tai could be built to serve different practical purposes. For example, it could be used as an observatory as is the one near Jianguomen in Beijing which dates back to the Ming and Qing dynasties. It could also be used for military purposes like the beacon towers along the Great Wall, to transmit urgent information with smoke by day and fire by night. Also on the Great Wall, there is a square tai at intervals of every 300 to 400 metres from which the garrison troops kept watch.