Plen-an-gwarry, or plain-an-gwary - the Cornish words mean simply "playing place" - was the Penwith name for a kind of structure once common across the west of the duchy. A roughly circular earth-banked amphitheatre, open to the sky, used for plays, sports, and any public business that needed a crowd. Some were repurposed Neolithic henges; many were medieval. Depending on whom you ask, 48 to 51 of them once existed across Cornwall, nine with surviving above-ground remains and another nineteen identified only by old place-names. Only two survive nearly intact: the Plain in St Just in Penwith, and Saint Piran's Round at Perranporth. Walk into either, and you can still feel the shape of the audience that once filled the bank.

The St Just plen-an-gwary sits in the middle of the town, a circular green between the streets, ringed by a low grass-topped wall. It is roughly 38 metres across at its central playing area, old enough that no one really knows when it was first dug. Cornish wrestling tournaments have been held in it from medieval times into the present century. So have Cornish mystery plays, particularly the Ordinalia - a cycle of biblical dramas written in the Cornish language by an unknown monk in the late 14th century. The renewal of its outer wall was nearly finished by December 1878, the work funded as a relief project for miners thrown out of work by the closure of the local pits. The Plain is now central to the annual Lafrowda Day festival every July - a community celebration that fills the circle with music, costume, and the same kind of noise that once frightened Spaniards.

The Cornish miracle plays were noisy, bawdy, and entertaining - this is the testimony of contemporary observers, not modern romanticism. They were written in Cornish, which is to say in a Brittonic language closely related to Welsh and Breton, spoken across most of Cornwall in the Middle Ages and slipping out of daily use only in the 17th and 18th centuries. The plays were Christian instruction at their core - the Ordinalia tells the story of the Creation, Christ's passion, and the Day of Judgement - but they were also pure popular theatre. Music, broad humour, terrifying devils, comic angels. The audience stood or sat on the earth bank around the playing area; the actors performed in the centre, sometimes for hours at a stretch. The Ordinalia is still occasionally staged at the St Just Plain, in modern Cornish revivalist productions. The language did not, in the end, quite die.

The best-preserved plen-an-gwary anywhere is not at St Just but in the parish of Perranzabuloe, midway between Perranporth and Goonhavern on Cornwall's north coast. Saint Piran's Round - sometimes called Perran Round - is a substantial circular earthwork, larger and more dramatic than St Just's modest urban green. It was almost certainly purpose-built as a theatre and may sit on an older Iron Age enclosure; Cornish wrestling tournaments have been documented there since the Middle Ages. The geographer's eye still reads it easily on the ground: the circular bank, the entrances, the central performing space. Stand in the middle and clap once, hard, and the bank gives a faint dry echo back.

The map of vanished plen-an-gwarys is a map of vanished Cornish community life. The Long Sentry field southeast of St Mabyn church holds the probable site of the most northerly playing place. There is evidence that Bartinney Castle near Sancreed in west Penwith may have been an Iron Age plen-an-gwary used for Celtic fire festivals. Plain-an-Gwarry in Redruth was documented for wrestling tournaments. A field a mile northeast of Marazion still carries the name. Smaller sites near Wendron churchtown and Mellangoose hosted the famous wrestler Captain Thomas Gundry. At the foot of Bodmin Beacon is a circle that locals long believed to be a wrestling ring; many archaeologists now think it was a plen-an-gwary too. Most of these are gone - filled in for fields, built over for houses, lost to memory. But every July at St Just, when the Lafrowda crowd packs the Plain and the band starts up, you can hear what they all used to sound like.