A Sonnet with very irregular iambic pentameter
This poem takes the form of a Petrarchan sonnet. We know this because the poem is composed of 14 lines, the three quatrains (groups of four lines) followed by a rhyming couplet (two lines) at the end, and the regular rhyme scheme. As for the rhyme scheme, the poem looks like this: ABBA, ABBA, CDCD, EE, with an unusual rhyme at the end of line 12: "enemy" with "I."
This history and tradition of this form are important to this poem. Until Donne writes this sequence, sonnets were almost always about a speaker's love for a woman. Instead of writing a little love song to a lady, Donne decides that this would be an appropriate form for speaking to God. Think about that for a second – Donne’s speaker attempts to address God exactly as if he is telling a woman that he thinks she's beautiful. On one hand, there's an intimacy and genuine affection for God here, but on the other hand, you can also construe this as serious disrespect for God. This tension between an earthly, physical attraction and a more sacred, spiritual form of love, so perfectly represented in the context of the sonnet form, is central to the poem's meaning.
Now, the other cool thing about sonnets, and one always worth keeping in mind, is that sonnet tradition dictates that the 9th line is a sharp "turn" in the poem, where the speaker's language, style, or content is expected to change. What's more, it's often the case that the poem presents a problem before the turn, while the poem works out a solution after the turn. Now, with that in mind, check out line 9: here the metaphysical conceit (see "Calling Card") switches from the speaker-as-fortress to a much more personal, less abstract metaphor of a lover who is engaged to someone else.
Finally, there's the meter. The poem is in iambic pentameter (five groups of an unstressed syllable followed by a stressed syllable). As a quick look over the poem will show, this clearly isn't what's going on for every syllable, but the effects of changes tend to be case-specific. Take the first line for instance: "Batter my heart" starts the poem with a strongly stressed first syllable, where a more regular iambic meter would start unstressed. Here, the idea is that Donne starts with a bang – this poem, like the action that the speaker asks for, is aggressive and unusual.