Aside from the base, traditional Chinese structures were mostly constructed of wood, which made them more susceptible to corrosion. The Cultural Revolution as well as foreign invasions over the last few centuries also saw mass destruction as well as looting. The Porcelain Tower for example was destroyed during the Taiping Rebellion. Religious and cultural artifacts, especially large metal objects, were taken down for smelting during the Cultural Revolution. Most of the large city walls which used to guard Chinese cities were torn down to make room for construction. Sometimes simply because there weren't any resources to spare for repairs. Today Chinese archaeology is primarily one of tomb hunting, as these were built using stone, and aside from looting, left untarnished by human turmoils.