Every evening, before dusk, the magician went to his study, a luxurious crowded room of very old books and furniture, and sat at his crystal ball. Every night he invoked its magic and through the ball, checked carefully the bird's eye view of leagues and leagues of the frontiers of the Kingdom. What he wanted was to be visible from a great distance, even at the height of the flight of a hawk. In this way he could cover large amounts of territory in a short time.
One afternoon, when he was beginning to get impatient, he saw a large troop formation, training and taking positions on a hill. They were carrying lots of heavy equipment, including catapults and crossbows. They had units of archers and soldiers and, though less numerous, also cavalry. There were transport and logistics units that accumulated numerous carts in the rear. Most of these troops were equipped to travel long distances relatively quickly. He could also guess a complete unit of sappers. A few of them had begun to dig trenches.
Everything indicated that the troops were making forts on that hill. Given the number of archers they had, they would need long lines of trenches, which would be a lot of work. But with a unit of sappers, they could solve it in a relatively short time.
The magician went to get his spell book. He reviewed and revised multiple pages until he found what he needed: a spell to transform the ground.