I have a question about sieges that always baffled me: How do you take a castle by treachery? As the besieger how can you identify and contact possible traitors in the castle?

Good question.

According to Aeneas Tacticus, author of the 4th century BCE best-seller How to Survive under Siege, key targets for treachery are social minorities, exiles, the families of hostages, and foreign mercenaries.

This is doubly true for any of them who are working as gatekeepers or guards, because those are the people who are physically closest to the enemy and could be communicated with more easily. 

But in terms of how you contact them, you get close to the walls and shout up to people, or you send in written letters, or you set up a parlay and use the opportunity to have your envoy talk to people face-to-face, or you send in a spy, etc. 

How does the idea that medieval warfare was seasonal match up with the idea that it was mostly focused on sieges? What happened if a siege started to drag into the harvesting season? Wouldn’t it make sense to just wait until the “off season” and then launch an attack on your unsuspecting foes who are all out tilling their fields?

Well, sieges and seasonal warfare had strong interactions: the foks inside would hope that supply problems would make the larger army go away before their stores inside ran out, the folks outside would try to feed themselves as best they could on the surrounding areas. But it’s also true that because of the size disparity between a castle garrison and an army, you could downsize your army to just enough men to keep them bottled in if the levies were needed in the field (this is another reason, btw why kings started to rely more on professional soldiers).