Around 230 AD, the Kushans were defeated by the Sassanid Empire and replaced by Sassanid vassals known as the Indo-Sassanids. During the Sassanian period, the city was referred to as "Kapul" in Pahlavi scripts. Kapol in the Persian language means Royal (ka) Bridge (pol), which is due to the main bridge on the Kabul River that connected the east and west of the city. In 420 AD the Indo-Sassanids were driven out of Afghanistan by the Xionite tribe known as the Kidarites, who were then replaced in the 460s by the Hephthalites. It became part of the surviving Turk Shahi Kingdom of Kapisa, also known as ''Kabul-Shahan''. According to ''Táríkhu-l Hind'' by Al-Biruni, Kabul was governed by princes of Turkic lineage whose rule lasted for about 60 generations.