City improvement of a guard tower, that gets an attack in combat when the city is attacked, this can be manned by an archer you create. By picking Fortress you get this build option. The tower gets a bonus, when a hero who picks defender is at the city when under siege.
Hero picks governor and city gets a production bonus (I guess this is in game now with -unrest), an increase in the kingdoms fame or only when he is stationed in a city.
New units like a trebuchet, war tower on wheels, etc
Mercenaries come to your city randomly based on fame and a roll. They can be any type of unit that any one of the sovereigns can make. It's a rare chance they show up.
Option to upgrade an outpost so you can station an arrow unit in it, when combat happens at the tower square.
Traveling merchant, who will show up rarely to offer goods you can't find.