Coordinated destruction of about ten river bridges, meant to funnel non-flying hunters into predictable crossings.
Word reached Carl as he crossed The Covered Bridge that maybe ten bridges over the river had been knocked down. It wasn't nearly as many as he had hoped for, but the logic held: Hunters who couldn't fly or teleport would be funneled toward whichever crossings remained standing. He was also counting on word of the demolitions leaking to the hunters themselves, so they'd route accordingly.
The scheme dovetails with the rest of his preparation — hunter-triggered mines along the road and, once Princess Donut claims the Large Ursine Settlement, its guards turned to defending the city at the bridgehead.
The strategic demolition that shapes where the hunters can cross the river.