In your diplomatic dealings with other players, insist on clarity, even if it requires a lot of back and forth yammering. This reduces the possibility of future misunderstandings.
However it also increases the possibility that people will get sick of PMing with you and ignore you entirely. It is a rather fine line.
If your about to lose your castle to a much larger force then you can handle and you still have a nearby province, break the siege early and retreat.
If they storm the castle then anyone who survives the battle will die anyhow. If you break the siege then those that retreat will appear in nearby provinces you control.
DO NOT do this if your castle is surrounded by enemy provinces as retreating will again mean curtain death.
Also, stealthy units can sneak in and out of sieged castles you control.