Ima, Soko ni Iru Boku, Tenkuu no Escaflowne Recommendations
Protagonist who is the ordinary schoolchild was transported to another dimension. He have to participate in a cruel war. There is either magic or high technologies. I can't say there is the same atmosphere but both anime are sad and made in realistic style without kawaii-elements. Also, there is a flying fortress in both anime.
Two series where a girl / boy is involved in a war for power, being teleported to a world where a country wants to govern everything else, deceiving others by making them believe in their ideologies by force. Also a girl with a mysterious necklace will be the key. Two worlds where they show the suffering of fighting, of how hopes will continue even in adversity, how greed and revenge give rise to tragedy.