Warfare was nearly constant in Wales both before and after the Norman conquest. Of course, the Normans didn’t actually conquer Wales–only parts of it–until the final defeat of Llywelyn in 1282.
In the years since 1066, however, the native Welsh princes and kings had lost out to the conquering Normans. Deheubarth, the southwestern region of Wales, was flatter and more accessible than the northern areas, and had been of particular interest to the conquerers. They had successfully overrun much of it by 1136, but in that year, the time was ripe for rebellion:
“By 1136 an opportunity arose for the Welsh to recover lands lost to the Marcher lords when Stephen de Blois displaced his cousin Empress Matilda from succeeding her father to the English throne the prior year, sparking the Anarchy in England.
The usurption and conflict it caused eroded central authority in England. The revolt began in south Wales, as Hywel ap Maredudd, lord of Brycheiniog (Brecknockshire), gathered his men and marched to the Gower, defeating the Norman and English colonists there at the Battle of Llwchwr.” http://en.wikipedia.org/wiki/Gwenllian_ferch_Gruffydd
One of the lords of Deheubarth, Gruffydd ap Rhys, saw an opportunity to regain what he’d lost in the last few years and journeyed to meet with Gruffydd ap Cynan of Gwynedd, his father-in-law, to enlist his aid in the revolt.
But Maurice of London and other Normans took Gruffydd’s absence as an opportunity to lead raids against the Welsh. Needing to defend her lands, Gwenllian raised an army, which was then routed near Kidwelly Castle. The Normans captured Gwenllian and beheaded her. Two of her sons, Morgan and Maelgwyn, also died (one slain in battle, one captured and executed).
When the two Gruffydds heard about Gwenllian’s death and the revolt it inspired in Gwent, Gwenllian’s brothers, Owain and Cadwaladr, invaded Deheubarth, taking Llanfihangel, Aberystwyth, and Llanbadarn. http://en.wikipedia.org/wiki/Gwenllian_ferch_Gruffydd
Following these events, the Battle of Crug Mawr occurred in October, 1136. “At Crug Mawr, two miles outside Cardigan, the Welsh forces were confronted by Norman troops drawn from all the lordships of South Wales. The Normans were led by Robert fitz Martin, lord of Cemais; Robert fitz Stephen, constable of Cardigan Castle; and William and Maurice fitz Gerald, uncles of Gerald of Windsor.
After some hard fighting, the Norman forces were put to flight and pursued as far as the River Teifi. Many of the fugitives tried to cross the bridge, which broke under the weight, with hundreds said to have drowned, clogging the river with the bodies of men and horses. Others fled to the town of Cardigan, which however was taken and burned by the Welsh though Robert fitz Martin successfully managed to defend and hold the castle; it was the only one to remain in Norman hands at the end of the rebellion.” http://en.academic.ru/dic.nsf/enwiki/1530079
Unfortunately, both Gruffydd ap Rhys (of Deheubarth) and Gruffydd ap Cynan (of Gwynedd) died in 1137, the former in battle or otherwise irregular circumstances, and the latter of old age. Anarawd succeeded to his father in Deheubarth and Owain to Gwynedd.
The Chronicle of the Princes (the Red Book of Hergest) has this to say:
“And after joining battle, with cruel fighting on every side, the Flemings and the Normans took to flight, according to their usual custom. And after some of them had been killed, and others burned, aand the limbs of the horses of others broken/ and others taken captive, and the greater part, like fools, drowned in the river, and after losing about three thousand of their men, they returned exceedingly sorrowful to their country. After that, Owain and Cadwalader returned, happy and rejoicing, to their country, having obtained the victory honourably/ with an immense number of prisoners, and spoils, and costly garments and arms.”