I, like an usurp'd town, to another due, Labour to admit you, but O, to no end. Reason, your viceroy in me, me should defend, But is captived, and proves weak or untrue. (lines 5-8)
See the "Detailed Summary" for more, but it's worth noting here that it's hard to tell who the good guys are and who the bad guys are. To whom is the town due, and who usurped it in the first place? If reason is God's viceroy, did God capture it, or did some other party take it away? We'll see this issue again in the next quote of this section, and it's a major thematic strain in the poem.
But am betroth'd unto your enemy, (line 10)
When we try to understand wars, it is central to recognize the identities of the parties at war. But, in the battle over the speaker's soul, we can't figure out who's fighting against God. This can be frustrating for us as readers, although we suspect this is the same crisis that the speaker suffers: he doesn't know who has captured him, and he also doesn't know who exactly to get in touch with to help him, as evidenced by his confusion over the nature of the Trinity. (See the discussion of the "three-personed God" in the "Detailed Summary" and "Quotes" in the "Religion" theme).
Take me to you, imprison me, for I Except you enthrall me, never shall be free, (lines 12-13)
Here, the speaker seems to bring back the castle siege metaphor one last time, and rekindles the debate we had earlier about who captures (or imprisons) the town in the first place. Again, the speaker refuses to make things clear: first asking God to imprison him, but only so that he can be free. This passage reminds us again that violence and aggressive behavior aren't necessarily bad things in this poem, so long as it's God who's doing them.