Cantref Mawr was a cantref in south-west Wales

Wales

Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...

. It was of strategic importance in medieval Wales as the location of the main seat of the princes of Deheubarth at Dinefwr

Dinefwr Castle

Dinefwr Castle is a Welsh castle overlooking the River Tywi near the town of Llandeilo, Carmarthenshire, Wales. It lies on a ridge on the northern bank of the Tywi, with a steep drop of several hundred feet to the river. Dinefwr was the chief seat of the Principality of Deheubarth...

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Cantref Mawr means "The Great Cantref". Each cantref in medieval Wales was divided into cwmwdau, and a typical cantref would be made up of two or three cwmwd. Cantref Mawr was made up of seven cwmwdau, which made it one of the largest cantrefs in Wales. Cantref Mawr was an upland area, bounded by the rivers Tywi

River Tywi

With a total length of the River Towy is the longest river flowing entirely within Wales, and is noted for its sea trout and salmon fishing.The river rises within of the Teifi on the lower slopes of Crug Gynan in the Cambrian Mountains and, flowing through the Towy Forest, forms the border...

, Teifi

River Teifi

The River Teifi forms the boundary between the counties of Ceredigion and Carmarthenshire in south-west Wales for most of its 75 mile length, flowing into the sea below the town of Cardigan. The catchment of the river is estimated to be 1,008 square kilometres yielding an average flow at Glan...

 and Gwili

River Gwili

The River Gwili is a river in Carmarthenshire, Wales. A tributary of Wales' largest river, the River Towy, it's headwater is found east of Llanllawddog, in the Brechfa Forest. It runs west, through Llanpumsaint, to its confluence with the River Duad, just south of Cynwyl Elfed village...

. The area at this period was covered in dense scrub which caused difficulties for invading armies, so that the Cantref Mawr often provided a secure refuge for the princes of Deheubarth when facing an invasion.