Over the course of 250 years, the throne slowly tried to integrate the most strategically and economically important regions—i.e. Lower Burma, Tenasserim, northernmost Irrawaddy valley—into the core by appointing its governors in place of hereditary rulers. In the 12th and 13th centuries, for example, Pagan made a point of appointing its governors in the Tenasserim coast to closely supervise the ports and revenues. By the second half of the 13th century, several key ports in Lower Burma (Prome, Bassein, Dala) were all ruled by senior princes of the royal family. However, the escape of Lower Burma from Upper Burma's orbit in the late 13th century proves that the region was far from fully integrated. History shows that the region would not be fully integrated into the core until the late 18th century.