Onomatopoeia refers to words that sound like what they mean. One of the clearest examples of this in the poem is the word "clank," which refers to the sound of the golden chains that the king is wearing. When pieces of metal hit together, they make a sound that sounds something like "clank."
One simile in this poem is where it says that "it passed, like a glorious roll of drums." To me, this is saying that the sounds sound like the roll of drums -- they are powerful and strong.