The Marches Border

The Welsh Marches (Welsh: Y Mers) is an imprecisely defined area along the border between England and Wales in the United Kingdom. The precise meaning of the term has varied at different periods.
The English term Welsh March (in medieval Latin Marchia Walliae) was originally used in the Middle Ages to denote the 'marches' between England and the Principality of Wales in which Marcher Lords had specific rights, exercised to some extent independently of the King of England. In modern usage, "the Marches" is often used to describe those English counties which lie along the border with Wales, particularly Shropshire and Herefordshire, and sometimes adjoining areas of Wales. However, at one time the Marches included all of the historic counties of Cheshire, Shropshire, Herefordshire, Worcestershire and Gloucestershire.
In this context the word march means a border region or frontier, and is cognate with the verb "to march," both ultimately derived from Proto-Indo-European *mereg-, "edge" or "boundary".   (Wikipedia)

After the fall of the Roman Empire, which occupied southern Britain until about 410 A.D., the area which is now Wales comprised a number of Romano-British kingdoms, one of which was Powys in the east. Over the next few centuries southern and eastern Britain was conquered and settled by the Angles, Saxons and others to become the kingdom of Mercia. About 820 A.D., Powys conducted raids and campaigns​ east into Mercia in retaliation to its expansion westward. This led to Wat's Dyke being constructed. a boundary earthwork extending from the Severn Valley near Ostwestry to the Dee Estuary.
As Mercia's power grew, Offa's Dyke was built between 757 and 796 A.D. by Offa, the Anglo-Saxon king of Mercia, to better delineate the separation between Wales, in the west, and Mercia. 
The Dyke was a stronger and longer earthwork boundary that still exists, and best seen at Knighton, close to the current England and Wales border.
​Both Dykes sometimes ran within a few yards of each other but never more than three miles (5 km) away. Wat's Dyke was originally of better construction, and considered to be strategically more sophisticated than Offa's Dyke.

The story of The Archer's Diary takes place roughly in the blue area.

Wat's Dyke — photo by Peter Craine

It consists of the usual bank and ditch of an ancient dyke, with the ditch on the western side, meaning that the dyke faces Wales and by implication can be seen as protecting the English lands to the east. The placement of the dyke in the terrain also shows that care was taken to provide clear views to the west and to use local features to the best defensive advantage.

Offa's Dyke — photo by Chris Heaton

art by Paul0599