Timber was the chief building material in Han architecture. It was used for grand palace halls, multi-story towers, multi-story residential halls, and humble abodes. However, due to wood's rapid decay over time and susceptibility to fire, the oldest wooden buildings found in China (i.e. several temple halls of Mount Wutai) date no earlier than the Tang dynasty (618-907). Architectural historian Robert L. Thorp describes the scarcity of Han-era archaeological remains, as well as the often unreliable Han-era literary and artistic sources used by historians for clues about non-existent Han architecture. What remains of Han-dynasty architecture are ruins of brick and rammed earth walls (including aboveground city walls and underground tomb walls), rammed earth platforms for terraced altars and halls, funerary stone or brick pillar-gates, and scattered ceramic roof tiles that once adorned timber halls. Sections of the Han-era rammed earth Great Wall still exist in Gansu province, along with the Han frontier ruins of thirty beacon towers and two fortified castles with crenellations. Han walls of frontier towns and forts in Inner Mongolia were typically constructed with stamped clay bricks instead of rammed earth.