From the book "The Battles of Wales" by Dilys Gater, comes this account of Gwenllian. I'm not suggesting that I'm related, but this is evidently one of the Gwenllian's from where the matrinomic surname Gwenlan originates:

Gwenllian, daughter of Gruffudd ap Rhys of Deheubarth

In an effort to wage war afresh on the Norman usurpers who were still challenging him even in his own kingdom, the Lord of Deheubarth departed for the north with his eldest son, to ask for assistance from his father-in-law Gruffudd. He was still away when news was brought to Gwenllian that English reinforcements for the depleted force of the Norman lord Maurice de Londres, at Kidwelly Castle, had landed on the coast of Glamorgan. Her husband had made no arrangements of the action to be taken if these troops should arrive, so rather than allow them to proceed from the coast to their destination, Gwenllian herself led out a force to intercept them, accompanied by her two younger sons, Maelgwn and Morgan

'Mounted on one of the sturdy surefooted galloways, of which Gruffudd kept many ready at all times for such sudden emergencies, although not the most stately of warsteeds, yet the best species of cavalry which the country afforded, the gallant lady rode forth,' we are told by an admiring writer

Her small army proceeded from the forest of Ystrad Tywi to Kidwelly, making many detours to avoid being seen. She intended to surprise the English forces, and led her men past the looming towers of Kidwelly Castle to a spot some two miles further on, where she called a halt at the foot of Mynydd y Garreg, with the River Gwendraeth flowing before them. Here she waited while scouts went out to ascertain the position of the English force. They returned with the news that the English had not yet reached Kidwelly Castle, but were on the march, and Gwenllian sent a large detachment to intercept them, while the rest remained with her

This is the area on a map from about 1885, with an aerial view from now:

Here, however, her plans went horribly wrong. The reinforcements, under the leadership of a Welsh traitor called Gruffudd ap Llywelyn, had not only managed to evade the troops she had sent to intercept them, but had followed a roundabout route to the heights of Mynydd y Garreg, and two days after she had taken up her position by the river, the full force of English burst over the top of the hill in a deadly attack. Simultaneously, Baron Maurice de Londres thundered from the castle at the head of his own men, so that Gwenllian and the remnants of her army were trapped.

Gwenllian personally led her forces against these overwhelming odds, and legend paints a gallant picture of this lovely princess, hair streaming beneath her helmet, sword in hand, encouraging her men by her own example as well as by her exhortations, even as her forces were cut to pieces around her. The end was inevitable. The battle did not last long, and Gwenllian saw her son Maelgwn slaughtered at her side while he was attempting to protect her. Most of the Welsh army fell before the might of the Norman numbers, and the rest were herded together as prisoners, Gwenllian among them suffering from a wound she had received.

The Baron de Londres, far from displaying any chivalric appreciation of a noble and courageous foe, and a woman at that, ordered that Gwenllian should be immediately executed. His order was carried out on the spot. While the Normans howled jeers and her stricken son Morgan watched in horror, accompanied by the other Welsh captives. Baron de Londres and his crony, the renegade Gruffudd ap Llywelyn, gloated over the downfall of their wretched foe. Gwenllian's head was hacked from her body — but she had earned herself by her gallantry and courage, a unique place in Welsh history.

This old song was sung when the warrior princess was a baby in her fathers court. "Sleep on" says the song, while Gruffudd ap Cynan was fighting the Normans. Her turn to carry on the struggle would arrive all too soon.

Sleep, Gwenllian

Sleep, Gwenllian, my heart's delight

Sleep on thro' shiv'ring spear and brand,

An apple rosy red within thy baby hand;

Thy pillowed cheeks a pair of roses bright,

Thy heart as happy day and night!

Mid all our woe, O! vision rare!

Sweet little princess cradled there,

Thy apple in thy hand thy all of earthly care.

Thy brethren battle with the foe,

Thy Sire's red strokes around him sweep,

Whilst thou, his bonny babe, art smiling through thy sleep

All Gwalia shudders at the Norman blow!

What are the angels whispering low

Of thy father now

Bright babe, asleep upon my knee,

How many a Queen of high degree

Would cast away her crown to slumber thus like thee!

Traditional Welsh Song