It's less about the model itself than the ability to quickly make one. If he can whip up a model of one city quickly, he can do it with another, such as the capital.
It's also not as immediately obvious as the aesthetic is european medieval/renaissance fantasy, but at least in Japan castle towns were intentionally designed in labyrinthine ways for defense purposes. While European castle towns and cities can also be confusing, this is more to do with organic growth and sprawl in relation to the landscape as opposed to a deliberate urban planning prioritizing the confusion of an invader.
Thus with that context a map isn't just strategically helpful but an intentionally kept secret.
I see, thank you
They are overly reliant on magic. Sure, it's convenient but in reality humans can do much more without magic than they do with it. What do you mean the kid has to make an abacus by himself? You have the design so you can do it without magic. And cobblestone roads too, do these people not know how to cut stone or make bricks? (︶︿︶)＝凸
PS can someone explain to me why the model of the city would be treasonous? I can understand it being dangerous if it falls into the wrong hands because it makes it easier for enemies to plan an invasion but why would it be a threat to the royal family? Do they not make maps?