A simple suggestion for trenches would be to use an irrigation graphic and change the color from blue to dark brown, or maybe gray. At civ scales, trenches would be mere lines, and there would quite a few of them, not just a ring trench around a region or a single trench along a border. The irrigation graphic with the color change would be an easy way to simulate a trench complex without going to all the trouble of creating a whole new graphic. But I would start with an irrigation graphic that has the irrigation channels fairly widely spaced, or maybe thin them out some, so the amount of trenches are not overwhelming.