I first encountered this poem in middle school, and it blew my mind wide open to see that haiku (something I associated with gentle philosophical nature imagery) could be employed to convey the stark, sharp imagery of prison life. The poet deftly builds tension and narrative in a series of haiku formations ending with a forceful proclamation. "See this? See this? See this? ... Hear THIS." It's masterful.
The opening haiku/stanza is a marvel of word choice, the way he blurs the nouns and verbs and adjectives to put the reader on edge and demand close attention. Look at all the nouns and adjectives that could be verbs here: guard, tower, [sun]set, convicts, rest. By the time we get to lizards, they seem to have a verbish quality, too.
It was also one of the first poems I encountered whose musicality is unrelenting. The first stanza sets up the slithering soft "i" sound that will carry the remainder of the poem: glints, convicts, lizards. There's also unity in the profusion of "r" sounds just in the first stanza: Eastern, guard, tower, rest, lizards, rocks. By the time we reach the forceful AIN'T in the closing stanza, we've already been doused with long "a" sounds that could almost hold the narrative by themselves: A.M., jailhouse, graves, bare, flakes, aches, rays, making, AIN'T, square. Meanwhile, the "ah" sounds take us the full length of the poem from convicts to job. But boy is musically isolated; the tall boy of the poem, whoever he is, stands alone.