Lady diamond is a Scottish ballad, as most  ballads.

It is about a tragic love story and it is one of the most popular topic of ballads. It is about a princess who fell in love with a kitchen boy but the king killed him. 

All the charachters are just sketched: the reader can understand  their  role and the narrator uses simple and concrete language to help  memorization. Ballads were an oral form of poetry.

As for the structure, the ballad is organized into thirteen stanzas and the reader can see the ballad consists of narration and dialogue. 

There are lots of repetitions:  for example the word "king" or the phrase "she loved him" and every line has lots of alliterations, as for example "go call to me my merry men all".

There are also rhymes (fame-name; scorn-corn; greet-meet; ...).