The exact dates of the Lydian conquest are unknown, but it must have taken place between Cyrus's overthrow of the Median kingdom (550 BC) and his conquest of Babylon (539 BC). It was common in the past to give 547 BC as the year of the conquest due to some interpretations of the Nabonidus Chronicle, but this position is currently not much held. The Lydians first attacked the Achaemenid Empire's city of Pteria in Cappadocia. Croesus besieged and captured the city enslaving its inhabitants. Meanwhile, the Persians invited the citizens of Ionia who were part of the Lydian kingdom to revolt against their ruler. The offer was rebuffed, and thus Cyrus levied an army and marched against the Lydians, increasing his numbers while passing through nations in his way. The Battle of Pteria was effectively a stalemate, with both sides suffering heavy casualties by nightfall. Croesus retreated to Sardis the following morning.