Lorca celebrated many aspects of his country in his work, and not the least of these is Spain's long-standing literary tradition. "Romance Sonambulo," then, uses a very, very old form of poem called the "romance" in Spanish (row-MAN-say), or "ballad" in English. This form stretched back to the Middle Ages in Spain, and was very regular both in terms of form and meter. See, it was passed down largely orally, as very few people could read back then. So it was easier to remember things if they were put to a regular rhyme and rhythm.
The conventional Spanish romance consists of eight-syllable lines, with every other line having an assonant, or vowel, rhyme. In this way, the rhymes of a romance are more casual than exact.
Check it out: in the Spanish version of the poem, Lorca writes "Verdes ramas" (VER-dehs RAH-mahs) and then the next rhyming line ends with "en la montaña" (mon-TAH-nyah). So, just reading that out loud will tell you that the "ah" vowel sound is the same, but the rhyme is not exact, the way "damas" (DAH-mahs) would rhyme with "ramas" for example.
Lorca's choice of form was no accident. He wanted to celebrate a form of poetry that was uniquely Spanish, all tied up with the country's history. At the same time, though, he wanted to put his own stamp on the form. By using dream-like imagery to talk about a gypsy girl, Lorca was really breaking with the historical conventions of the form. Most romances tended to deal with national, historical events, not marginalized people like gypsies, and certainly not in the surrealist way that Lorca does in this poem. "Romance Sonambulo" turns the Spanish romance on its head, honoring the past, ushering in the future.
Adding another layer to all of this, of course, is the fact that we're reading this poem in translation. For more on that, and how the translator's choices can affect the form, rhythm, and sound of the poem, go check out "Sound Check."