[Taqwus]

Commanders in castles are vulnerable to assassination unless the castle is under siege, which makes sense as the gates are shut and there's no commerce in or out to provide cover. (Hm, does this make sense for ethereal assassins? Or those with illusion power?)

Province defense is not really 'units' per se, but a measure of organization of the provincial citizens. Since it does not manifest itself as units and commanders outside of battle, it is immune to assassination and farstriking spells such as Flames from the Sky; it also does not suffer from such niceties as Burden of Time. It is, however, generally not awe-inspiring and rather importantly the owner cannot set formations or orders.