Question:   Which play, set in  Ancient Britain , was created by Shakespeare?

Context articles: Silures
Vacomagi
Venicones
Brigantia (ancient region)
Cornovii (Caithness)
Thomas Jenkins (headmaster)
Epidii
Agricola (book)
Carnonacae
Cymbeline

The Silures were a powerful and warlike tribe or tribal confederation of ancient Britain, occupying what is now south east Wales and perhaps some adjoining areas.
 They were bordered to the north by the Ordovices; to the east by the Dobunni; and to the west by the Demetae.
The Vacomagi were a people of ancient Britain, known only from a single mention of them by the geographer Ptolemy c. 150.
 From his general description and the approximate locations of their neighbors, their territory was the region of Strathspey, including that part of the northern coast of Scotland.
 Ptolemy says that their towns or principal places were called 'Bannatia', 'Tamia', "Pinnata Castra", and 'Tuesis'.
The Venicones were a people of ancient Britain, known only from a single mention of them by the geographer Ptolemy c. 150 AD.
 He recorded that their town was 'Orrea'.
 This has been identified as the Roman fort of Horrea Classis, located by Rivet and Smith as Monifieth, six miles east of Dundee.
 Therefore, they are presumed to have lived between the Tay and the Mounth, south of Aberdeen.
 Andrew Breeze has suggested that the tribal name probably means "hunting hounds".
 A slightly differing etymology, "kindred hounds", identifies the name with "Maen Gwyngwn", a region mentioned in the "Gododdin".
Brigantia is the land inhabited by the Brigantes, a British Celtic tribe which occupied the largest territory in ancient Britain.
 The territory of Brigantia which now forms Northern England and part of The Midlands covered the majority of the land between the River Tyne and the River Humber forming the largest Brythonic Kingdom in ancient Britain.
 It was recorded by Ptolemy in the 2nd century AD to extend sea to sea, from the Irish sea on the west coast to the North sea in the east
The Cornovii were a people of ancient Britain, known only from a single mention of them by the geographer Ptolemy c. 150.
 From his description, their territory is reliably known to have been at the northern tip of Scotland, in Caithness.
 Ptolemy does not provide them with a town or principal place.
Thomas Jenkins was the headmaster of the King Edward VI Grammar School in Stratford-upon-Avon in England starting in 1575.
 As such, his claim to fame is that William Shakespeare is considered likely to have been one of his students.
 No school records from the period survive; however, Jenkins is believed to have been of Welsh extraction, and a Welsh schoolmaster Sir Hugh Evans features in Shakespeare's play "The Merry Wives of Windsor".
 Jenkins would have taught Shakespeare Latin grammar and literature.
 It has been speculated that Jenkins taught, and awakened in Shakespeare, an interest in the legendary history of the British Isles, and as such influenced Shakespeare to create plays featuring this material, such as "King Lear" and "Cymbeline".
The Epidii (Greek: Επίδιοι) were a people of ancient Britain, known from a mention of them by the geographer Ptolemy c. 150.
 "Epidion" has been identified as the island of Islay in modern Argyll.
 Ptolemy does not list a town for the Epidii, but the Ravenna Cosmography (RC 108.4) mentions "Rauatonium", which is assumed to be Southend.
The Agricola (Latin: "De vita et moribus Iulii Agricolae" , lit.
 On the life and character of Julius Agricola) is a book by the Roman historian Tacitus, written  AD 98 , which recounts the life of his father-in-law Gnaeus Julius Agricola, an eminent Roman general and Governor of Britain from AD 77/78 - 83/84.
 It also covers, briefly, the geography and ethnography of ancient Britain.
 As in the "Germania", Tacitus favorably contrasts the liberty of the native Britons to the corruption and tyranny of the Empire; the book also contains eloquent and forceful polemics against the rapacity and greed of Rome.
The Carnonacae were a people of ancient Britain, known only from a single mention of them by the geographer Ptolemy c. 150.
 From his general description and the approximate locations of their neighbors, their territory was along the western coast of modern Ross-shire.
 Ptolemy does not provide them with a town or principal place.
Cymbeline , also known as Cymbeline, King of Britain, is a play by William Shakespeare set in Ancient Britain and based on legends that formed part of the Matter of Britain concerning the early Celtic British King Cunobeline.
 Although listed as a tragedy in the First Folio, modern critics often classify "Cymbeline" as a romance or even a comedy.
 Like "Othello" and "The Winter's Tale", it deals with the themes of innocence and jealousy.
 While the precise date of composition remains unknown, the play was certainly produced as early as 1611.