Carnedd Llewelyn lies in the middle of the main north-east to south-west ridge of the Carneddau, between Carnedd Dafydd to the south-west and Foel Grach to the north. A short subsidiary ridge links it to Yr Elen to the north-west. Its position means that any route to this summit involves a long walk. It can be climbed from Gerlan, above Bethesda, taking the path following Afon Llafar then continuing to the summit of Yr Elen before following the short ridge to Carnedd Llewelyn. Another path starts from Helyg on the A5, taking the track to the reservoir then following the slopes above Craig yr Ysfa to the summit. An alternative is to reach it by following the main ridge, either from Pen yr Ole Wen or from Foel-fras.
The summit is a flat, boulder-strewn plateau, similar to several of the other mountains that lie in the southern Carneddau. The cliffs below the ridges are well-known rock climbs, notably Ysgolion Duon (meaning "black ladders") and Craig yr Ysfa. The flat plateau means that during winter and spring, significant accumulations of snow and blizzards occur on frequent occasions. The mountain has also been noted for its long lying snow patches in recent years, with old snow patches known to survive on its southern gullies well into July. The highest lake in Wales, Llyn Llyffant lies to the lee of the summit of Carnedd Llywelyn.
Carnedd Llywelyn means "Llywelyn's cairn" in Welsh. It is widely believed that Carnedd Llewelyn and the neighbouring Carnedd Dafydd are named after Llywelyn ap Gruffudd and his brother Dafydd ap Gruffudd, the last independent prince of Wales, respectively. An alternative theory is that the twin peaks are named after Llywelyn the Great, an earlier prince of Gwynedd, and his son and successor, Dafydd ap Llywelyn. Other sources cite a combination of the above, i.e. Llywelyn the Great and Dafydd ap Gruffudd.
The spelling of the name is also controversial. Carnedd Llewelyn is the form used by the Ordnance Survey, the mapping agency for Great Britain, and other sources. In Wales the spelling Carnedd Llywelyn predominates (it is used on the website of the Snowdonia National Park Authority, for example); this is also the form preferred by most Welsh writers, among others. Many authoritative works, from other study groups, also use the Welsh form. The Welsh personal name Llywelyn, from which the mountain's name is derived, is always spelt thus in the Welsh language, although the forms Llewelyn and Llewellyn are found in older English-language sources.
^For example, Dewi Tomos, Eryri (Gwasg Carreg Gwalch, 2005); Colin Marsh, The Mountain Walker's Guide to Wales (Gwasg Carreg Gwalch, 2002); or Harvey Superwalker Waterproof Map, Snowdonia, the Glyderau and the Carneddau (Harvey Maps, 2003).
^G. H. Howe & P. Thomas, Welsh Landforms & Scenery (Macmillan, 1963); Physical Names for the Map of Britain: Discussion (The Geographical Journal (102), 1943); or M. O. Hill, D. F. Evans & S. A. Bell, Long Term Effect of Excluding Sheep from Hill Pastures in North Wales (The Journal of Ecology (80), 1992).