Spies are the best means to keep a province loyal (being much cheaper than garrisoning the province with an army but as much efficient). One more use of the spies the manual speaks of is sending them worming into a besieged castle to open the gate and deliver it to the besiegers. But, to my mind, this function is useless. I mean, I see the main objective of the besiegers avoiding casualties as much as possible while trying to get hold of the castle. But if the besieged garrison is quite large, it is likely to hold out only for a year (two as a maximum), so you will get the castle with minimal casualties by just sitting and waiting. If it is so small to be able to survive the siege for 4 years or more, you can storm the castle again with minimal (though more palpable than in the first case) losses. So why use the spy? It will take him so long to get his job done (that is if he ever does it) that you can do without his help; thus, saving manpower is insignificant. Does anyone use the spies despite this logics?