An upgrade to outposts from the Feudal age or Castle age using the blacksmith or University. Perhaps it can be a civ bonus that you get for free or a castle tech maybe.
The fortified outpost triggers a notification when an army of a certain size passes by it. You can use the arrows on the unit to select the number of units needed to trigger the fortified outpost. This would be a situational unit and not so overpowered.