Laddertop Vol 1

Laddertop by Orson Scott Card. A Sci-Fi manga rated All Ages, published in English by Seven Seas.

Giant towers called Laddertops were left behind by mysterious aliens, and only children are small enough to maintain the alien technology inside. Best friends Robbie and Azure are training to become Laddertop crew, but the closer they get, the more secrets they uncover.