The other man in the alliance against Edwin (besides Penda) was Cadwallon, king of Gwynedd. He is not to be confused with his predecessor, Cadwallon Lawhir. This Cadwallon actually grew up with Edwin at the court of Deira, where the two boys were playmates and friends. Their friendship grew into a sometimes intense rivalry and then exploded into warfare when they both became king of their respective kingdoms.
Jealously probably had a lot to do with Cadwallon’s motivation to fight against his longtime friend, Edwin, although the fact that Edwin wanted to take over his kingdom also probably prompted Cadwallon to fight back. But Edwin proved the more able strategist (at least his army had more men standing when the battle was over), and Northumbria claimed Gwynedd for its own stable of territories, which by that point was growing quite large.
Meanwhile, Cadwallon was licking his wounds, having escaped into Ireland. He traveled about the two islands, trying to raise anti-Northumbria forces but not having much luck. He did find enough willing men, however, to land in Dumnonia and raise a Mercian siege, routing the invading Mercians and their leader, Penda, in the process. It was at this point that Penda and Cadwallon became allies, though the terms of their alliance are shrouded in the mists of lost history. Whether the alliance was grudging or friendly or forced or mutually respectable is not known. What is known is that the two leaders fought together against Edwin and killed him at Hatfield Chase in 633.
Full of victory and full of himself, Cadwallon stayed on in Northumbria, living off the fruits of the land and his victory. While there, Cadwallon survived a charge from Osric, Edwin’s cousin, and then met his match in Oswald at the Battle of the Wall, at Heavenfield, near Hexham.
Cadwallon died as he had lived, a hero on the battlefield and a leader of his people. The kingdom of Gwynedd would long survive independently because of his strong leadership.