The Khan’s Circle. Survival through the Ages, #6

Waking up in the middle of a 100-mile wide wall of horsemen, 15-year-old Robert finds himself trapped inside the Great Hunt of Genghis Khan. Thousands of Mongolian riders are slowly shrinking a massive circle, herding every living thing-predators and prey alike-toward a central killing field. Robert isn't being hunted by soldiers yet; he is being herded like an animal. To survive, he must use the prehistoric stealth and tracking skills he learned in the past to "break" the circle, a feat of survival that no beast or man has ever achieved.
Robert's impossible escape from the tightening perimeter immediately catches the eye of Genghis Khan, who sees the boy's "ghost-like" movement and mechanical intuition as a gift from the spirits. Captured but respected, Robert is forced to use his modern "mechanic's brain" to improve the Horde's siege engines while secretly tracking the shimmering atmospheric "glitch" moving across the high steppes.
In a final, desperate ride across the open plains, he plunges through the anomaly and wakes up safely in his own bed in 2020.