What JimmyIowa said. My longest-serving A19 base uses a ~20 m deep moat to get the maximum fall damage on one fall for common low-level zombies. But fall damage aside, moats or trenches more generally are an effective way to funnel zombies. Your alternatives for funneling are basically walls, which won’t be very thick since you have build them up, or raised platforms, whose support pillars will be attacked semi-randomly. In fact as I think about it, a trench may be the most demolisher-resistant option, since fundamentally it’s a hole, and the demolisher just blasts out a bigger hole.