Lost a castle because of naiveness.
Ragnar and his hellhounds (about 800 men) were sieging my castle. I somehow managed to distract other lords and left Ragnar there alone. After a half-day of chase, with a nice and quick U-turn I ditched them and returned to Ragnar. He was already fighting with my garrisoned men (300 vs 100 or so I think). I joined and did beat him. Yet, siege wasn't over. To finish him off, I talked to him. Then I made the most stupid mistake.
I wanted to check my inventory before attacking so I clicked to end the conversation. Ragnar didn't let me. He asked whether I could lend my surgeon. I said what the heck, since I've never done this before. Sadly, I didn't know that this mini-quest took a few hours to complete. Game fast-forwarded, rest of the lords caught up and I left my Curin Castle feeling like a sucker.
Damn Nords and their trickery.