de Mortimer led Edward’s campaign in mid Wales, including the besieging and capture of Dolforwyn in 1277. As a reward Edward granted him the castle, along with the lands of Ceri and Cedewain.
Roger de Mortimer was a powerful Marcher baron, based at Wigmore, Hereford, a senior figure of the increasingly powerful Mortimer family who held extensive lands in Wales and England.
[Later Mortimer Marcher Lords would hold even more sway over English, Welsh and Irish politics. For example; de Mortimer’s grandson, also called Roger Mortimer, 1st Earl of March, held estates in the Welsh Marches and Ireland (where he held the pre-eminent role of Lord Lieutenant of Ireland), later leading the Marcher lords in revolt against Edward II (the Despenser War of 1321). Following Edward IIs removal from power and death the de Mortimer descendent ruled England for a number of years before Edward III reclaimed the throne, whereupon de Mortimer was hung.]
As part of the first Baron de Mortimer's reward for removing Llewelyn, the King granted him a charter in 1279 to establish a new market town in Cedewain. He chose to site it close to the western edge of his territory (possibly a choice made to prevent a rival lord establishing a market at nearby Caersws), in a narrow valley upon the River Severn, next to the riverside chapel of St. Mary (hence - Llanfair-yn-nghydewain or ‘St. Mary’s church in Cedewain’).
As a medieval market town Newtown seems to have been fairly unremarkable. Its boundaries and layout followed the typical medieval plan, filling in the north and eastern part of the current town centre, its main thoroughfares forming the medieval T-shape with Broad Street forming the stem of the T running north to a ford, and later a wooden bridge across the river. Development west of the town centre was constrained by a large deer park, part of the Newtown Hall manor. The basic layout of the town centre, its streets, the size of its shop frontages, street names and landmarks, and the date of the market (Tuesdays) remain with us today.