"But the felons of this land made him believe this lie, for it is easy to deceive the loyal heart." -Iseult
"The cowards would remove from the King's side all those who love him; they have succeeded and now mock him." -Tristan
I felt a lot of pity toward King Mark because as Tristan and Iseult are saying all these remarks about others deceiving him, when it is actually them who are deceiving the king. This irony is so obvious to the reader as Tristan and Iseult know that they are manipulating the king because he has a "loyal heart" and they are "now mock(ing) him."