They Do!
And you can keep building new ones anywhere in that grid after the first attack in that battle instead of expanding on the existing ones with more facings in other directions.
I'm not a big fan of that.
As it stands now you can build all over that grid during each battle that takes place there with new entrenchments in different areas even if you just fought a battle there last turn.
I do not have an issue with that per say. A unit over time will improve their fighting positions over time. But I DON'T think it should be automatic dig it... Should be a choice with the fatigue penalty. Also need a new template or 2 of dug in types. I would like to see something with over head protection if dug in a LONG time. Shallow grave type fox holes... the sort you can dig in 20 minutes if you have too would be nice. Well depending on soil type or if its frozen.