A Horatian ode is one in which a poet chooses a meter and rhyme scheme and sticks to this format throughout the poem he is writing. It is a predictable, harmonious form that does not jar or startle the reader.
Marvell's "An Horatian Ode" is an occasional poem, which means it was written to celebrate a public occasion, not express a private emotion. The occasion of this 1650 poem is Cromwell's victorious return from violent military campaigns in Ireland.
Marvell uses a very regular, balanced, and rhythmic format by choosing iambic tetrameter, a line of eight syllables with the emphasis on the second syllable to create a da-DUM, da-DUM rhythm. He uses rhyming couplets throughout, such as "wall" and "hall" and "peace" and "cease." This regularity of form creates a sense of harmony.
Marvell also creates a sense of balance through the literary device of antithesis, which is the putting together of opposites. For example, in the following two couplets, peace and war are juxtaposed and set in opposition to each other:
So restless Cromwell could not cease
In the inglorious arts of peace,
But thorough advent’rous war
Urged his active star.
Likewise the illusion of theater and the reality of the beheading of Charles I are put together:
That thence the royal actor [Charles I] borne
The tragic scaffold might adorn,
While round the armed bands
Did clap their bloody hands.
Marvell also balances his imagery
, which is description using any of the five senses of sight, sound, touch, taste, or smell. Bloodthirsty, violent images coexist against peaceful imagery. For example, the image of a peaceful, creative Cromwell planting his "private gardens" with sweet-smelling "bergamot" is placed against Cromwell as "angry Heaven’s flame," a harsh, destructive image.
Beneath this seeming balance, however, the poem expresses a sense of unease at Cromwell's violent tendencies, ending with the warning that Cromwell will continue to rule in a bloody way:
The same arts that did gain
A pow’r, must it maintain.