Question: The concept of treason can be dated back to the early Roman republic, but was defined by nebulous criteria, it would be the common laws of this time period which would most directly influence those customary in the court of King Arthur, a legendary British leader who, according to medieval histories and romances, led the defence of Britain against which invaders in the late 5th and early 6th centuries AD?

Context articles: Budic II of Brittany
Treason in Arthurian legend
Halfdan
St Twrog's Church, Bodwrog
King Arthur
Eucheria
Battle of Badon
Roman-Volscian wars
Cynllo
Ogham

Budic II (Medieval Latin: "Budicius" ; Welsh: "Budig" or "Buddig "; c. 460 – c. 550 ), formerly known as Budick, was a king of Cornouaille in Brittany in the late 5th and early 6th centuries.
 He was the father of Hoel Mawr and is probably to be identified with the Emyr Llydaw ("Emperor of Brittany") and King Nentres who appear in Arthurian legend.
 Upon his death, his kingdom was usurped by Macliau, king of the neighboring Veneti.
The concept of treason can be dated back to the early Roman republic, but was defined by nebulous criteria.
 Frederic William Maitland, author of The History of English Law Before the Time of Edward I, has said that "treason is a crime [with] a vague circumference and more than one centre."
 Early French and Anglo-Saxon laws for the prosecution of persons deemed traitorous were inspired by, and in some cases, directly pulled from, late Roman and Germanic conceptions of the crime.
 It would be the common laws of this time period which would most directly influence those customary in King Arthur’s court — assuming its existence is founded in more than the legends and fables of medieval romances.
Halfdan (Old Norse: "Halfdan" , Old English: "Healfdene" , Medieval , Proto-Norse: "*Halbadaniz", "half Dane") was a late 5th and early 6th century legendary Danish king of the Scylding (Skjöldung) lineage, the son of king named Fróði in many accounts, noted mainly as the father to the two kings who succeeded him in the rule of Denmark, kings named Hroðgar and Halga in the Old English poem "Beowulf" and named Hróar and Helgi in Old Norse accounts.
St Twrog's Church is a small rural church at Bodwrog in Anglesey, North Wales.
 Built in the late 15th century in a medieval style, some alterations have been made but much of the original structure still remains.
 It has two 15th-century doorways (one later converted into a window) and some 15th-century windows.
 The bull's head decoration used on the church denotes a connection with the Bulkeleys of Beaumaris, a prominent north Wales family over several centuries.
 Set in a remote part of the countryside in the middle of Anglesey, it is dedicated to St Twrog, who was active in the late 5th and early 6th centuries.
 The church's tithes were paid for at least two hundred years to Jesus College, Oxford, which has historically strong links with Wales, and the college at one point built a house for the priest who served St Twrog's and a neighbouring parish.
King Arthur is a legendary British leader who, according to medieval histories and romances, led the defence of Britain against Saxon invaders in the late 5th and early 6th centuries AD.
 The details of Arthur's story are mainly composed of folklore and literary invention, and his historical existence is debated and disputed by modern historians.
 The sparse historical background of Arthur is gleaned from various sources, including the "Annales Cambriae", the "Historia Brittonum", and the writings of Gildas.
 Arthur's name also occurs in early poetic sources such as "Y Gododdin".
Eucheria was a Roman poet who was active sometime in the late 5th to early 6th centuries AD.
 She is notable for being, along with Sulpicia, a rare example of a Roman female satirists.
 One poem of hers survives, an epigram that ridicules a lower-class suitor.
 The poem is composed of 16 elegiac couplets, which play on the theme of courtship between classes with the juxtaposition of objects of high and low quality, such as a "noble Punic tunic and an awful shaggy rag."
 Vocabulary in the poem suggests that she may have been from Gaul, particularly the court in Aquitania.
 Attempts to identify her with historically attested women named Eucheria active in Late Antique Gaul have been inconclusive.
The Battle of Badon (Latin: "Bellum in monte Badonis" or "Mons Badonicus", Welsh: "Cad Mynydd Baddon" , all literally meaning "Battle of Mount Badon" or "Battle of Badon Hill") was a battle thought to have occurred between Celtic Britons and Anglo-Saxons in the late 5th or early 6th century.
 It was credited as a major victory for the Britons, stopping the encroachment of the Anglo-Saxon kingdoms for a period.
 It is chiefly known today for the supposed involvement of King Arthur, a tradition that first clearly appeared in the 9th-century "Historia Brittonum".
 Because of the limited number of sources, there is no certainty about the date, location, or details of the fighting.
The Roman-Volscian wars were a series of wars fought between the Roman Republic and the Volsci, an ancient Italic people.
 Volscian migration into southern Latium led to conflict with that region's old inhabitants, the Latins under leadership of Rome, the region's dominant city-state.
 By the late 5th century BC, the Volsci were increasingly on the defensive and by the end of the Samnite Wars had been incorporated into the Roman Republic.
 The ancient historians devoted considerable space to Volscian wars in their accounts of the early Roman Republic, but the historical accuracy of much of this material has been questioned by modern historians.
Saint Cynllo is a British saint, who lived in the late 5th and early 6th centuries.
 His feast day is on 17 July.
Ogham ( ; Modern Irish ] or ] ; Old Irish: "ogam" ] ) is an Early Medieval alphabet used to write the early Irish language (in the "orthodox" inscriptions, 1st to 6th centuries AD), and later the Old Irish language (scholastic ogham, 6th to 9th centuries).
 There are roughly 400 surviving orthodox inscriptions on stone monuments throughout Ireland and western Britain; the bulk of which are in southern Munster.
 The largest number outside Ireland are in Pembrokeshire, Wales.