Medieval warfare was never honourable- it was a bloody mess and chaotic most of the time. Besieging a castle involved an ultimatium to the defenders- surrender or be slain- no prisoners would have been taken if the besiegers broke into the castle. Unless you where a knight worthy of a random- in which case you where treated quite well and returned once the ransom was paid. However, if you where a French knight at Agincourt you would'nt have been so lucky. The English took so many French prisoners that they outnumbered themselves, and scare of food supplies, the English butchered the French prisoners instead. Likwise, when the Normans conquored England- rebellion was crushed merclessly- peasants massacred and their homes burnt to the ground.
There would be some exhamples of chivalry. Richard the Lionheart was killed by a French boy wielding a cross bow from the walls of this castle he was besieging. Dying from gangerine Richard had the boy brought to him and compensated the boy for killing a King, giving explict instructions that thye boy's life be sparred- unlike the other castle defenders. However, one chronicler records that when Richard was dead one of his mercanary captains had the boy flayed alive anyway.
If anything, modern warfare, with the Geneva Convention and strict rules protecting civilians is a lot more "honourble" than it ever has been before.