Darius I or Darius the Great (c. 549 BC October 486 BC), was a Zoroastrian Persian Shahanshah (Great King) of Persia. He reigned from September 522 to October 486 BC as the third Achaemenian King, and was called by some "the greatest of the Achaemenid kings". He managed not only to "hold together the empire" (to use his words), but also to extend the empire founded by Cyrus the Great in all directions; east into the Indus valley, north against the Saka tribes, and west into Thrace and Macedon. His reign lasted 35 years and completed the work of his Achaemenian predecessors. Under Darius and the generation he belonged to, Achaemenid Iran became the greatest power in the world--arguably, Earth's first sole superpower in relation to the then known world. However, the successful expansion of the empire was not Darius' only important achievement. He also centralized administration of his empire and encouraged the development of cultural and artistic activity as demonstrated by his building projects at Susa and Persepolis.