Those English words are by good ol' Mrs Hemans (here
) and the Welsh are by J. Ceiriog Hughes (here )
.

They're based on a much much older poem by Llywarch Hen - "Canu Heledd" (The Song of Heledd) - about the aftermath of a battle in 577 A.D. The warrior Ceawlin led an army across what was then Gwent and is now Wiltshire towards the Severn valley. The Severn men were overcome and the invaders arrived at Pen Gwern - Shrewsbury - which was the seat of Cynddylan. They smashed that as well. The bits of the poem (which is epic and gratifyingly long) can be found here.

sian