User blog comment:Mrob27/The moat experiment/@comment-69.11.30.207-20120411164842/@comment-4728735-20120414194208

Try making the trench top 3 blocks wide and add an overhang, and then make a one block wide trench in the bottom. The three block wide top prevents them from walking over it, the bottom pit allows you to walk through and push the collected mobs down