- City walls, which the player can freely place and layout; and,
- City gates, which open and close as units pass through them and which a player can close or lock to force re-routing.
There are various styles of city walls (wooden palisades, stone walls) and gatehouses that match the decor of a particular city, but another strong benefit is the placement of military units on battlements to make the cities much more defensible.
I hope that Tilted Mill will look at what The Settlers 6 did with city walls -- especially their free-placing nature (as players are really able to help define the boundaries of a city superbly) -- and innovate off of this excellent example.
Thanks for listening!