Caerphilly Castle is Wales’s largest castle and, after Windsor, the second-largest in Britain — a 30-acre concentric fortress ringed by some of the most elaborate artificial water defences ever built in medieval Britain. It is also, according to local legend, patrolled after dark by a heartbroken noblewoman in green.

The castle’s origins are defensive rather than romantic. From 1268, the Anglo-Norman marcher lord Gilbert de Clare, 7th Earl of Gloucester, threw up the fortress at speed in direct response to the growing power of Llywelyn ap Gruffudd, Prince of Wales, who twice tried to destroy the works before they were finished. The result was Britain’s first large-scale use of concentric (“walls-within-walls”) design, a template later borrowed for Edward I’s castles in Gwynedd.

After Llywelyn’s death the fortress was softened into a grand residence, eventually passing to Hugh Despenser the Younger, Edward II’s notorious favourite, who remodelled the Great Hall in high style before his own downfall and execution in 1326. Civil War-era slighting with gunpowder later left the south-east tower leaning further off vertical than the Tower of Pisa — now the castle’s best-known visual signature.

The ghost story layered onto this history centres on the “Green Lady,” said to be Alice de la Marche (also rendered Alice of Angoulême, or Alice de Lusignan), wife of Gilbert de Clare. As the tale goes, Alice began a secret romance with a visiting nobleman, Gruffudd the Fair, while her husband was away campaigning. De Clare’s men hunted Gruffudd down and hanged him; on hearing the news, Alice reportedly died of grief, and her spirit returned to Caerphilly to walk the ramparts on moonlit nights, dressed in green — a colour writers variously explain as an allusion to the castle’s ivy or to her husband’s jealousy.

It’s worth being direct about the evidence problem here: several genealogical write-ups note that the real Alice de Lusignan never actually visited Caerphilly Castle at all, which marks the Green Lady as a romantic legend built around a real name rather than a documented historical haunting. A second, older strand of local folklore attaches the Gwrach y Rhibyn — a Welsh banshee-like hag with bat wings, said to shriek before a death — to the marshy ground and moat around the castle; this is a piece of broader South Wales folk-creature lore rather than a claim specific to any recorded event at Caerphilly.

Taken together, the Caerphilly material sits firmly in the folklore/anecdotal category: retellings trace back to local legend and popular ghost-story anthologies rather than witness testimony, formal investigation, or photographic evidence. Readers weighing how much stock to put in it may find our guide to how ghost hunting evidence gets evaluated useful context. None of that undercuts the site’s value to Weird World’s readers, though — Caerphilly earns its place as one of the most architecturally significant medieval castles in Britain regardless of whether the Green Lady walks its walls, and the atmosphere of its flooded moats, leaning tower and vast empty wards does a great deal of the ghost story’s work for it.