sentences:  According to archaeological evidence, the partial burning of Persepolis did not affect the Persepolis Fortification Archive tablets, but may have caused the eventual collapse of the upper part of the northern Fortification wall that preserved the tablets until their recovery by the Oriental Institute's archaeologists.Thousands of clay tablets, fragments and seal impressions in the Persepolis archives are a part of a single administrative system representing continuity of activity and flow of data over more than fifty consecutive years (509 to 457 BCE).These records can throw light on the geography, economy, and administration, as well as the religion and social conditions of the Persepolis region, the heartland of the Persian' Great Kings from Darius I the Great to Artaxerxes I.
  
  Persepolis administrative archives are the single most important extant primary source for understanding the internal workings of the Persian Achaemenid Empire.