Silver door. Emily Rodda. 2012.

Rye is willing to risk everything to save his brothers, lost in the terrifying world beyond the Wall of Weld. Sonia is determined to find and destroy the Enemy who is sending the ferocious flying beasts called skimmers to ravage the city. The companions barely escaped with their lives during their first thrilling adventure beyond the golden Door. But they won’t give up. Their desperate quest must continue. Deep in the ancient heart of the Keep of the Weld, two more doors await them.