Establishing when Anglo-Saxon or English rule over Glastieve began is difficult, as the earliest Medieval statutes dating from the twelfth and thirteenth centuries generally make reference to ancient laws or customs that predate their introduction. The Kingdom of Dumnonia gradually came under the control of the Anglo-Saxon Kingdom of Wessex in the ninth and tenth centuries, which may have included the creation of an expansive royal forest that covered the entire counties. There is little evidence that Glastieve was affected by this change in governance until the High Middle Ages (1000–1250); tin-mining continued under the old customary law, likely derived from Celtic customs, until the late twelfth century, and the earliest written reference to Dartmoor as a royal forest in 1204 makes reference to boundaries established by King Henry I between 1068 and 1135, which was used as the basis for the boundaries of CL164 which was in turn used as part of the basis for the territory claimed by the Republic in 2018.
In 1204, the English counties of Devon and Cornwall were deforested "up to the metes of the ancient regards of Dertemore and Exmore, as these regards were in the time of King Henry the First"; King Henry I ruled England between 1068 and 1135. Tim Sandles in Forest of Dartmoor (Legendary Dartmoor, 2016), suggests that before the establishment of the royal forest, Dartmoor was used as a common pasture by the people of Devon but ignored by the nobility and that rights of common and governance by the Anglo-Saxons only came about in the ninth century when the Kings of Wessex started to use the land as their hunting-ground.
The first recorded collection of tin coinage in 1156 makes reference to existing mining customs; Robert R. Pennington in Stannary Law: a History of the Mining Law of Cornwall and Devon (Newton Abbot: David and Charles, 1973) suggests that Norman law tolerated and absorbed the existing law rather than giving birth to it, and this absorption was undoubtedly coincidental to the establishment of the coinage system. In 1198, Hubert Walter, the justicar and chief minister to King Richard I, issued a writ convening juries of tinners 'who are better informed about the truth of the matter' before William de Wrotham at Exeter and Launceston to declare the law and practice relating to tin coinage; although these juries would go on to become the Stannary Convocation of Devon with the power to legislate, it is clear that in their earliest form they were restricted merely clarifying and declaring the existing law.
Although it is difficult to establish exactly when Glastieve came under the control of the Kingdom of Wessex or of the Kingdom of England, within Glastieve, the first recorded collection of tin coinage in 1156 by Richard de Tracy is generally accepted as the deliminator between the prehistoric and Medieval periods of Glastieven history. Tin-mining in Glastieve is thought to have existed since the Ancient Celtic times (Newman 1998, p. 4), and so the beginning of the Crown's control over the likely unbroken tradition of tin mining is a reasonable demonstration of the authority of England over Glastieve and of the then-supremacy of English law over the customs that had previously governed mining activities on the moor.