Caerphilly Castle is everything that a castle should be: turreted, moated and steeped in gory history. Half-close your eyes and you can still see men in armour scattering peasants as they gallop through its gates.

But it is a ruined castle some two miles to the south that has really captured the imagination of the town’s school-age historians. The decaying remnants of Castell Morgraig sit high on a heavily wooded ridge overlooking Cardiff and the Bristol Channel, but the castle’s profile is so low that even regulars of the nearby pub are unaware of its existence. For much of the year its only visitors are the grazing cattle who scuff the 13th-century walls with their hooves.

However, it is Morgraig’s very anonymity that has helped fire the interest of pupils at St Cenydd comprehensive. Caerphilly Castle may be magnificent and second only to Windsor in size, but it offers relatively few challenges for the children who have grown up within its bailiwick. Almost everything that could be known about it - the significant dates, the height of the keep, even the degree of skew-whiffness of its Pisa-like tower - has been exhaustively documented.

By contrast, Morgraig has been loath to divulge its secrets. It is believed to have been the setting for the doomed stand of Llywelyn the Last against Edward II. But opinion is divided over whether it was the Normans or the Welsh who built its Sutton limestone walls, or even if the castle was ever completed.

The school’s involvement dates back to March of last year when Brian Davies, of Pontypridd’s Cultural and Heritage Museum, suggested that the mystery of Morgraig might be an interesting topic for study. Martin Williams, the head of history, visited the site and a remarkable curriculum success story began.

Mr Williams initially thought Morgraig might be a suitable subject for one Year 7 class to explore. The work would slot neatly into the space allocated to “Studying an historical issue or topic in a local context” (national curriculum extension unit 5a). Pupils would measure the walls, examine some of the historical and literary references and, after six weeks, move on to something else.

But 20 months later, some of those Year 7 pupils are still mulling over Morgraig’s mysteries, sixth-formers have been drawn into the project and the castle studies have spilled over into English, art and geography.

The project has also earned a pound;1,000 Welsh Heritage Schools Initiative Award and been recognised as a prime example of how new technology can be used to enhance humanities teaching.

Much of the credit for this success must go to Mr Williams. He is something of a rarity: an historian who is as interested in 21st-century technology as he is in 13th-century masonry.

He quickly appreciated that pupils would be happier as historical sleuths if the research involved digital cameras, laptops and the Internet. But he insists that it is the pupils who have repeatedly led the way. They seized the initiative by e-mailing distinguished historians and asking for their advice, creating web pages and “cutting” their own CD-Rom.

St Cenydd is not the first Caerphilly comprehensive to investigate Morgraig’s past - a former head of art at St Martin’s, Alan Thomas Gill, produced a model of how the castle might have looked in its heyday - but St Cenydd is the only school to go down such a hi-tech history path and capitalise so fully on the castle’s mystery.

“The question of whether the castle is Norman or Welsh has intrigued the pupils,” Mr Williams says. “English historians have tended to say it is Norman, while Welsh historians have claimed it for their own.”

So who did build the castle? The presence of dressed stones suggests Norman builders, but Year 9 pupils Laura Ogden and Richard Selway are sure it was a Welsh achievement. Richard points out that the strongest walls faced towards the Normans in Cardiff, while Laura notes the absence of a well (Normans, sensibly, always ensured their castles had one).

Mr Williams is more guarded: “If it’s Welsh it is unique, and the southernmost castle defending traditional Welsh land against the Normans. But, given the lack of documentation, I think the mystery will never be solved.” Judging by the smile on his face, that will be no bad thing. There is, after all, nothing less interesting than a completed jigsaw.

Further information about the project can be obtained from the school’s website: www.stcenydd.org.uk