Raid, siege and reinforcements.
Q: What would happen if I send a raid/reinforcements to a castle which I besieged?
A: You can't raid or send reinforcements to a castle besieged by yourself. You can however send reinforcements to your troops besieging the castle.
Q: What would happen if I besieged a castle which has my reinforcements?
A: Your reinforcements will merge with your siege. As long as your siege was successful. In case of a failed siege, nothing will happen to your reinforcements.
Q: What would happen if I raid a castle which has my reinforcements?
A: Nothing will happen to your reinforcements. It will just stay there chillin'.
Q: What would happen if I send a raid and a siege at the same time?
A: In case your raid hits first, your raiding troops will return to bring the spoil of war and your siege will siege the castle. Alternatively, if your siege hits first it will become reinforcements for your siege.
Anyone wanna add something or pointing out something wrong?
Some asked me these question and it's annoying to answer 1 by 1. >_>