Up to ten Soldiers can be garrisoned at the Keep and one Soldier per Treasury component within the Manor.
Having Soldiers employed allows the player to complete missions, which are sent by an Envoy acting on behalf of the Fealty. An Envoy will periodically attend the Village Center and a mission prompt will appear. As of the current build (1.0.7.0207), there is no other function for Soldiers.
Soldiers are equipped with a helmet and sword. They spend their time standing guard, patrolling the estate and using Training Dummies.