Castles and resources question

[Saxon]

I have built a castle next to the enemy capital, with castle. Do I steal a portion of his resources and he a portion of mine or do we get nothing from each other?

Why would I do such an odd thing? He has a large army of high strength units that make it next to impossible to break down the defenses and a third nation keeps attacking me when I lay siege. I decided to let the third nation carry the pointless siege, but wanted to keep the other nation bottled up. I am also attacking their dominion while I pull together a much better siege army. This is SP, which explains some of the odd behavior.

Anyway, am I reducing their resources or just sharing them?