Full Metal Mystery

Joshua thought this was just a different world with a little more black technology but no different from the previous life. At most, it was just a world war. Until he inherited an ancient castle from his uncle William... A magical library that is constantly moving, and a maze of corridors underneath that can lead to many places. What is the secret of this world? Six-story-high mechas fight against demons of the same size, space-based satellites bombard demon nests, and mechanical armies fight against magic. This is a world of technology and magic.