2 Our faulty weakness, and your matchless might:
faulty > blameworthy; guilty
3 For-thy, fair sir, yours be the damsel,
For-thy > Therefore
4 Who by her own law to your lot does light,
law > command; law light > fall
5 And we your liege men faith to you plight."
liege men > vassals, faithful followers
6 So underneath her feet their swords they marred,
marred > spoiled (i.e. broke or damaged as a sign of submission, if "swords" is taken literally); or: disgraced, debased (if "swords" is taken to mean their collective valour, which is debased by the act of laying their weapons at the victor's feet - particularly as they later break their word. In either case the use is forced)