Complex sentence: The poem tells us that Aneirin was present at this battle and, having been taken prisoner, was one of only two or four Brythonic survivors; he remained a captive until his ransom was paid by Ceneu ap Llywarch Hen.
Split into simpler sentences: The poem tells us that Aneirin was present at this battle and, having been taken prisoner, was one of only four (or two) Brythonic survivors. He remained a captive until his ransom was paid by Ceneu ap Llywarch Hen.