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Castell y Bere is as wild and remote as it was when it was first built by Llywelyn ab Iorwerth in the 13th century.

Its ruins stretch along the summit of a rocky outcrop on the eastern side of the Dysynni valley, looking out over the valley's green farmland which stretches out to the Cardigan Bay coast.

It's not one of Wales's most famous castles, being built by a native Welsh prince as opposed to the Normans or Edward I (Llywelyn ab Iorwerth is also known as Llywelyn the Great, though the castle was taken by Edward in 1283).

But sitting atop the ruins recently were two different sort of kings.

Perched on the stone ruins, with the huge mountain Cadair Idris rising up behind them, were Robert Plant and Brian Johnson, two of the biggest names in the history of rock music.

(Image: Sky UK Ltd)

Until recently, when he was forced to retire because of the threat of "total deafness", Johnson fronted AC/DC, who have sold 200 million albums worldwide.

And Plant is the singer of Led Zeppelin, who broke all records for the amount of money made from touring and redefined rock and roll excess.

He has a long history with Wales, which goes back his whole life. As a child, he would holiday in the country with his parents, and he has been interested in Celtic history for decades. At the height of Zeppelin's fame, they began to write their third album, Led Zeppelin III, in a small cottage immortalised in the song Bron-yr-Aur Stomp.

He also donated money towards a bronze sculpture of Owain Glyndwr which was unveiled at Pennal Church, near Machynlleth, in 2004.

Plant and Johnson met in rural mid Wales to talk about Plant's career for the Sky Arts series Brian Johnson: A Life on the Road.

They first meet on the Penmaenpool Bridge, which spans the Mawddach river between Dolgellau and Barmouth, with Robert Plant carrying a sword and wearing a sort-of robe.

"In 1973, Zeppelin made this strange movie called The Song Remains the Same," Plant tells Johnson.

"And each of us chose a kind of topic that we wanted to intersperse the music with so I chose to come up here. This is the very same apparel.

"This is the Mawddach Valley and the beautiful Mawddach river that goes out to the Cambrian coast there. It's beautiful, isn't it? This is called the Misty Mountains here."

(Image: Sky UK Ltd) (Image: Sky UK Ltd)

Plant gets a lot of inspiration for his lyrics and music from the Welsh landscape and mythology, or as Johnson puts it, "the mythology of all this Welsh stuff".

"From an early age my mum and dad brought me here," says Plant. "At school I was really interested in the different culture between the original tribes and those coming in from Europe. They just pushed all the Celts and original Celtic tribes back into these hills and so when I was at school I was intrigued by the difference between the two cultures, the history, the whole idea of mythology, stuff like that.

"Coming from the Black Country, this compared to the industrial Midlands was such a relief to everybody."

Gazing out over the low-lying land just before the Mawddach river reaches the sea, he says: "Look at this here, it's empty. Plenty of room for everybody. We've both lived in places where there's no change in the seasons. You get the sun tan lotion out and that's it. But in this place, you really know where you are in the whole cycle of life really."

Johnson - whose role as AC/DC singer was sensationally filled by Axl Rose of Guns n Roses so they could continue to tour - says: "Driving in, you know, from England and all, through these beautiful roads and the little villages and I'm wondering to myself 'Why have I not come here before?'"

(Image: Sky UK Ltd) (Image: Sky UK Ltd) (Image: Sky UK Ltd)

Plant is fascinated by Castell y Bere and the surrounding hills and valleys, described as "a place nearby that's been really special... over the years and a retreat from the madness of touring in the Led Zeppelin days".

He explains it to Johnson: "This is a valley that comes out into the sea near Tywyn, which is just north of Aberdyfi. And it's a fantastic old castle that Llywelyn, the great king of the principality of north Wales, this was one of his stomping grounds where he was trying to hold back the English. For many years he succeeded because down here below us, what's now pasture, once upon a time was actually the sea."

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He goes on: "I came up here after Led Zeppelin II, which we recorded mostly on tour because we played so much in America at that time, with Maureen [his wife] and my daughter, Carmen and a dog called Strider, and we rented a cottage that I used to go to when I was a kid, which is not far from here, and we began the writing of Led Zeppelin III up there.

"And it was great because it was so far away from the chaos of touring, you know what it's like. You guys kept going, we had to stop. The idea of going from those gigs that you know about, where there is complete turmoil, to come to these places gave us the kind of foundations for Led Zeppelin III, which was much more pastoral and acoustic based."

Johnson, blown away, says: "Look at this place, it's stunning. Stairway to heaven. We're going to talk about your life on the road but it's pretty hard to get started when you come up here and look at this view."

(Image: Bryn Roberts/Creative Commons) (Image: Daily Mirror) (Image: Creative Commons) (Image: Sky UK Ltd)

After Plant and drummer John Bonham - who met when Bonham approached Plant to tell him he needed "the world's best drummer" in his band - teamed up with guitarist Jimmy Page and bass player John Paul Jones, Led Zeppelin were born.

By the end of the 1960s and early 1970s, Led Zeppelin "had become so huge that they were now actually bigger than The Beatles, a feat up to that time thought impossible", the band's biographer Mick Wall told the programme.

How big are AC/DC? AC/DC has sold more than 200 million records worldwide. They have sold 71 million in the US alone, which reportedly makes them one of the five top-selling bands in American music history. Their Black Ice world tour between 2008 and 2010 is reportedly the sixth highest grossing tour of all time, making $495 million (£348 million). The only bands and acts to have had higher grossing tours are U2, The Rolling Stones, Guns n Roses, Coldplay and Roger Waters.

They broke all records for the amount of money made from touring, thanks in part to their physically huge manager Peter Grant, a former wrestler, who completely changed the way money from touring was allocated, so that it was now weighed heavily in favour of the band instead of the promoter, as it had previously been. They toured the US with their own private plane.

(Image: Mirrorpix) (Image: Sky UK Ltd)

"They didn't just live to excess, they were rewriting the rules of the road. They lived like the Romans without the lions," said Wall.

"By the end of their tours, their rider was 50 pages long. These guys lived like no rock stars had ever lived before - or since."

How big are Led Zeppelin? Led Zeppelin's biographer, Mick Wall, told Sky Arts that by the early 1970s, Led Zeppelin were "so huge that they were now actually bigger than The Beatles, a feat up to that time thought impossible". They broke records for revenue for tours. Despite having broken up in 1980 when John Bonham died, they are still making extraordinary amounts of money. In 2016, an economist testified in a lawsuit that songwriters Robert Plant and Jimmy Page had made nearly $60 million (£42 million) from their songs between 2011 and 2016 alone. It's estimated that they have sold 300 million records worldwide. In 1977, it's reported that 76,000 people attended one of their shows, then a world record for a single act show. But 200,000 reportedly came to the Knebworth Festival, where they performed in 1979.

John Bonham died after a heavy drinking session in 1980, which led to the end of the band and Plant going solo. He is still a solo artist today and has written songs in several genres, from 1980s synth pop to indigenous Moroccan music to bluegrass, for which he teamed up with Alison Krauss with huge success.

Who was Llywelyn ab Iorwerth? Llywelyn ab Iorwerth was the grandson of Owain Gwynedd and was born at Dolwyddelan castle at the end of the 12th century, around 1173. By 1200, he had control of Gwynedd and, later, established himself as the foremost ruler of 'Pura Wallia' - the areas of Wales not under the control of the Marcher lords. He was using the title 'Prince of the whole of North Wales' by 1200. By 1215, the castles of Carmarthen, Kidwelly, Llansteffan, Cardigan and Cilgerran had fallen to his alliance. He was now leader of the free princes of Wales and ruler of most of the nation. The next year, more princes affirmed their allegiance to him and in 1218 he built castles including Criccieth, Castell Y Bere, Dolwyddelan and Tomen Y Bala. He died in 1240.

Brian Johnson's A Life on the Road is now showing on Sky Arts and on Sky On Demand.