Gameplay mechanic idea: Road Tolls
In medieval Eurasia, road and bridge tolls were a fairly common way for settlements and feudal lords (i.e. robber knights) to grow rich and powerful, particularly when situated on lucrative trade routes.
This could be potentially implemented in-game too:
Every couple of days, the game spawns in a group of neutral NPC travellers on one end of the map, who then take the quickest route to the other end, where they despawn.
It's up to the player to chase them down, stop them and shake them down levy a fee for passing through their turf.
The player can build roads and bridges, or otherwise manipulate the geography to entice or force travellers to pass by their settlement (by way of quickest pathfinding).
However, watch out for guarded caravans - these guys will rather pay in steel than gold!
(Pictured: The city of Munich with its famous bridge over the river Isar. A major 'salt road' led through here, making the city rich.)