Identify examples of repetition and onomatopoeia in the provided poem excerpt.

Here are the lines from the poem that show repetitions:

a) Cannon to right of them,

b) Cannon to left of them,

c) Cannon in front of them

d) Flashed all their sabres bare, / Flashed as they turned in air

The word from the poem that shows the use of onomatopoeia is:
thundered