This poem sounds to us like a symphony orchestra. It has all kinds of different sounds, movements and tones. When the river is crashing through the caves, we imagine the pounding of kettledrums. Listen to those rocks crashing: "Huge fragments vaulted like rebounding hail" (line 21). That word "rebounding" has such a hollow, open sound that we can hear the pounding of the rocks even as they are being described. Then, when we travel through the gardens, we hear the soft sounds of the woodwinds. It's hard not to feel soothed by the tone of line 9: "Where blossomed many an incense bearing tree." These are calm, quiet moments. When you say the words out loud, they have the sound of a soothing, delicate instrument like a flute.
At the other extreme, the scary, flashing-eyed figure that appears at the end reminds us of the horns, sharp and brassy and startling. Listen to the way the words cut through the air at this moment: "Beware! Beware!"(line 49). These words are blurted out, quick and loud, like the sound of a trumpet blaring out a warning. The poem is a journey of sounds. It tries to use the effects of language as if they were the different parts of an orchestra.