Construction is a little bit easier with flying creatures and levitation.
Well, not sure I agree that a city is more likely to develop without walls. People like security, and just the presence of them gives off a feeling of security, and it's not unfounded.
He's not wrong, in a way. If the city is large, the castle itself doesn't need to be as fortified. But it doesn't hurt, and I don't buy that it breeds passivity. You still need to protect the entire city, and that's not going to be as impregnable as a highly fortified castle.
Him without magic power. Can we have one of these stories without an OP protagonist? Is that a concept that exists? Does that even work? Ok, sure, he's far less OP than most manga fantasy protagonists, but still.