From William Shakespeare’s Henry VI, part 1 (III: 4):
Scene: Paris – The Palace
Villain, thou know’st the law of arms is such
That whoso draws a sword, ’tis present death,
Or else this blow should broach thy dearest blood.
Blackstone in his Commentaries (IV. 124): “By the ancient law … fighting in the king’s palace … was punished with death.”