The Outcast Chronicles, book 1: Besieged
Rowena Cory Daniells2012

King Chald has laid siege to the Celestial City, home of the mystics. Scholar-warrior Tobazim went to the city seeking glory, but now he's trapped along with the rest of his people. Surrounded and outnumbered, Imoshen, leader of the mystics, accepts banishment for her kind.

Far from the siege, Ronnyn and his family live in hiding. He and his sister have grown up listening to their mother's stories of the Celestial City. He dreams of going there one day, unaware that his family will be hunted down and executed, if they don't leave with the rest of their people.