In the poem Donne uses the conceit, or elaborate metaphor, of the speaker's sinful heart as a besieged city. The speaker, who resides in this city, isn't asking for mercy or clemency from God; he wants him to come and ram down the doors of his sinful heart and overpower him. Donne also uses the metaphor of the speaker as a maiden betrothed to God's enemy and therefore in desperate need of being saved, hence the overwhelming sense of urgency about the poem—the air of crisis that hangs over almost every line.
Donne uses vigorous language to drive home his point. He uses verbs such as "o'erthrow," "break," "imprison," "ravish," and, of course, "batter," all of which convey the impression of God as some kind of conqueror. The speaker adopts the characteristics associated at the time with women. In keeping with the prevailing misogynist standards, he is weak and feeble, unable to free himself from the grip of his betrothed—sin—without God's help. He is every bit as dependent on God for his salvation as a woman at that time would've been expected to be on her husband.
In the final couplet the speaker expresses the paradox that he can only be free once God enthralls him—that is, imprisons him—and can only be chaste once God has ravished him. It's as if, having been freed from the besieged city of sin, the speaker willingly desires to submit himself to the complete domination of the Almighty, as he knows he lacks what it takes to live a sinless life on his own.