If you are in a fort that is under siege you CANNOT protect surrounding areas. Why ? Because you're under siege! So I think it makes sense for troops to be able to roam freely throughout the provinces that YOU CANNOT control.
In other news, you cannot psychically give orders to armies that are 200 miles away from your capital and have them immediately follow them.
Realistically, armies do not in fact have the ability to instantly engage all other armies in the same administrative unit of a country the moment they arrive there.
It is also quite impossible to load 1000 men into a single cog.
Typically, it is not feasible to create a brand new unit of 1000 horse-mounted mercenaries in 15 days.
Fortunately, none of these realism breaking mechanics are a problem because realism is not a meaningful argument.