Each stanza (or "sestet," the name for a stanza with six lines) in the ballad "The Highwayman" follows the same rhyme pattern, which is AABCCB. In other words, the first two lines rhyme ("trees ... seas"), as do the third and sixth ("moor ... door") and the fourth and fifth ("riding ... riding"). This regular rhyme pattern creates a steady, driving rhythm which echoes the fast-paced drama of the poem.
There are also other rhythmic techniques used in the poem. For example, the fourth and fifth lines are always shorter than the other four lines in each stanza. These shorter lines quicken the pace at the same point in each stanza, each time hastening the reader towards the conclusion of the action described in the first three lines.
Finally—and this last technique can be quite difficult to—you could look at the prosody of the poem. Prosody refers to the number of feet in each line. A foot, in poetry terms, is a combination of usually two or three syllables with one or two of those syllables emphasized. A poet will often repeat the same feet over and over again to lull the reader's voice into a pattern or rhythm of emphases. So, for example, "The Highwayman" is mostly made up of feet called iambs (two syllables, where the second is emphasized) and anapests (three syllables, where the third is emphasized). For example:
"The wind /was a torr/ent of dark/ness among /the gus/ty trees.
The moon /was a ghost/ly gall/eon tossed /upon cloud/y seas.
The road /was a rib/bon of moon/light o/ver the purp/le moor"
So the first of those lines is made up of an iamb, followed by three anapests, followed by two more iambs. This can seem a bit odd the first time you think about it, but the patterning of feet in this way really does add to the rhythm of a poem as much as, or more than, the more basic rhyme patterns outlined at the beginning of this response.