Trenches are okay, but raised earthen walls are actually better. If you just have trenches, trolls can stand on the edge and slam rocks at you. Smash a bunch of stuff up.

You can created raised "walls" by using the hoe to add stone. it will stretch the bottom level of the map (the mesh) upward. If you stand in one spot, the ground will raise up farther and farther and leave perfectly smooth sides. They'll still be sloped a bit, but can almost get them straight up and down.

https://steamcommunity.com/sharedfiles/filedetails/?id=2760270389

and the shot below is of an attempt to make "merlons" on the wall. My character is standing on a sloping ramp in the corner of the walls.

https://steamcommunity.com/sharedfiles/filedetails/?id=2760270446

The merlons were functional, not decorative. I used them when attacked by ice drakes, so I could hide behind them to block their shots. Pop out and shoot back at them with a bow.