The lost king

"After the invasion of Khul, thirteen-year-old Watt's only goal is to lie low and survive, but when he learns of his sister Melior's plan to escape from beneath the castle, Watt incites events that change the kingdom's destiny."@en
"Khul, a proud kingdom founded by slaves, is invaded. The king is slain, his family in hiding, and resistance brutally suppressed. The people being enslaved again wonder if the king's son is still alive and if he will return to save them?"@en
"The Slave City of Khul, a proud Kingdom founded by slaves, is invaded. The Slave King and his family slaughtered and resistance brutally suppressed. The former slaves are slaves once more. The slaves whisper a name that has not been heard or repeated since the City fell. Avtar. Avtar is their king. Thirteen year-old Watt is a Ghosty Boy, so called for the flour-covered uniform of The Baker's apprentices, and works in the ancient fortress of Slave City, a far-flung outreach of Empire. When Watt gets stuck cleaning the Baker's chimney, he falls into a labyrinth of corridors where he finds his si."@en