Complex sentence: Ivaylo was defeated by the Mongols and besieged in important fortress of Drastar and in his absence the nobility in Tarnovo opened the gates to Ivan Asen III. However, Ivaylo managed to break the siege and Ivan Asen III fled back to the Byzantine empire.
Split into simpler sentences: Ivaylo was defeated by the Mongols and besieged in important fortress of Drastar. In his absence the nobility in Tarnovo opened the gates to Ivan Asen III. However, Ivaylo managed to break the siege and Ivan Asen III fled back to the Byzantine Empire.