Upon his brother
Sultan Kaykaus's unexpected death in 1219/1220 Kayqubad was released from captivity and succeeded to the Seljuk throne as its new Sultan. In the Cilicia Campaign of 1225, Kayqubad reduced the
kingdom of Cilician Armenia to a vassal state. In 1221/1222 Kayqubad launched a
naval attack on Sudak which defeated the combined forces of
Rus and
Cumans. He attacked the Armenian Kingdom of Cilicia in 1221 taking the city of
Alanya from its governor, Kir Fard. , late 1220s. In 1227/1228, Kayqubad advanced into
Anatolia, where the arrival of
Jalal al-Din Mangburni, who was fleeing the destruction of his
Khwarezmian Empire by the
Mongols, had created an unstable political situation. The sultan settled
Turcomans along the
Taurus Mountains frontier, in a region later called
İçel. At the end of the 13th century, these Turcomans established the
Karamanids. The
Ayyubids, who were disturbed by the rapid expansion of Sultan Kayqubad I, especially in eastern Anatolia, took action against the sultan under the leadership of
Al-Kamil in
Egypt. In 1234 Kayqubad completely defeated the allied
Ayyubid forces. Afterward,
Harput expanded its borders further in the south-east Anatolia region by capturing
Siverek,
Urfa,
Harran and
Raqqa. The sultan defeated the
Artuqids and the
Ayyubids and absorbed the
Mengujek emirate into the sultanate, capturing the fortresses of
Hısn Mansur,
Kahta, and
Çemişgezek along his march. He also put down a revolt by the
Empire of Trebizond and, although he
fell short of capturing their capital, forced the
Komnenos dynasty family to renew their pledges of vassalage. At first, Kayqubad sought an alliance with his
Turkish kinsman
Jalal al-Din Mangburni against the
Mongol threat. The alliance could not be achieved, and afterward, Jalal al-Din took the important fortress at
Ahlat. Kayqubad finally defeated him at the
Battle of Yassıçimen between
Sivas and
Erzincan in 1230. After his victory, he advanced further east, establishing Seljuq rule over
Erzurum, Ahlat and the region of
Lake Van (formerly part of Ayyubids). The Artuqids of
Diyarbakır and the Ayyubids of
Syria recognized his sovereignty. He also captured several fortresses in
Georgia, whose queen sued for peace and gave her daughter
Tamar in marriage to Kayqubad's son,
Kaykhusraw II. Mindful of the increasing presence and power of the
Mongols on the borders of the Sultanate of Rum, he strengthened the defenses and fortresses in his eastern provinces. In 1237, the
Ayyubid sultan
Al-Kamil sent an envoy to Kayqubad in Kayseri seeking an end to hostilities. The envoy arrived too late, Kayqubad having died 31 May 1237. ==Architecture and culture==