I would like to see the ability to "dig in" and build fortifications on non-city squares - even inside enemy territory. Maybe this should require a special unit to be able to do and maybe take a few turns to complete. Taking over a fortified position should yield some extra benefit, such as being able to repair the fortification and hold it as your own at minimal cost.
Building a fortification on a trade route might even be a way to disrupt said trade route, or maybe your could tax those traveling the route in order for "safe passage".
Any other ideas regarding fortifications out in the wild?
edit: darn sorry for the duplicate thread https://forums.elementalgame.com/334395