Prison my heart in thy steel bosom's ward, But then my friend's heart let my poor heart bail; (9-10)
Oh, look. Another cliché. Now our speaker is saying that he's some kind of love prisoner. But, again, there's something really ugly going on here. When he calls his mistress' heart (a.k.a. her "bosom") a "steel," jail cell ("ward"), we get a sense of how cold and harsh he thinks she is. We also notice that he's more interested in protecting his friend from his mistress than anything else. Here, he seems to be offering himself up to her as some kind of sacrifice so she'll stop hurting his buddy.