It's historical fiction based on myth and also history, which means it's basically an amazing fantasy story. Theseus, founder-hero of Athens has amazing adventures. There's battle, ancient kingdoms, magic, ancient rites, messages from gods, incredible heroism, pride and honor, tragedy, wonder. And it's all from the point of view of Theseus, whose Ancient Greek way of thinking is different enough from our own that you might as well be reading science fiction. It is so, so good.