Grand Bishop Nicholas was a thousand yards high in the sky and saw that metal castles and watchtowers were everywhere in western town. If he flew within their range, he would definitely be shot down.
That was not even considering the ten story magic tower. If there was an anti-flight region and the holy priests were not able to control their flying technique, it was entirely possible for them crash to their deaths in the city.
The city also consisted of magic cannons which were much more accurate than magic catapults. The magic tower was entirely capable of providing the coordinates of the armies on the outside so that the magic cannons in the city could fire with extremely high accuracy.
