Carl's request to destroy every bridge across the floor's river except one, leaving a single crossing.
Studying Katia's map of the floor, Carl asked whether she could help organize something for him. Katia sighed that she would do anything he asked, to a point, and Carl grinned: he wanted all the bridges knocked down — all except one.
The river bisects the floor from the northeast corner to the southwest like a slithering snake, and Katia's map places at least a dozen bridges, probably more, most of them only approximately located. With crawlers streaming south to escape Zockau and the Hunters due to spread across the map within a day or two, funneling everything through a single crossing shapes the whole floor's traffic. The bridge Carl chose to spare carries the main road out of Zockau toward the Large Ursine Settlement.
Carl's opening strategic move on the sixth floor: control the river crossings, control the hunt.