Just some thoughts from a Noob

[FaceLess]

By default, movement to your own province with a castle will always result in having the troops inside the castle. You can change the move order to move and patrol to have the troops directly help in defending the province. Note that the option "move and patrol" only becomes available after you first give the move order and only if the target province contains a castle you own.
In provinces with a castle, only commanders (and the troops they lead) with the patrol order will defend the province(together with pd). Thus troops on defend, preach, research, forging, casting, etc will never help to defend the province.