The boulders surrounding the hamlet of Fachwen and the village of Brynrefail are a bit of a mixed bag really. Some super classics are to be found with much mid-grade climbing, probably stemming from the wealth of history in these very accessible boulders. Craig yr Undeb, for example, is often known as Union Rock: a meeting place for trade unionists over a hundred years ago and would've not that long ago been alongside the main road into Llanberis.
Likewise Lion Rock is a very popular crag for beginner groups and many is the day dozens of youngsters in bright helmets can be seen across the area; not that this should affect the visiting boulderer much as most of the bouldering isn't on the crag proper, with much of the circuit scattered around the hillside between Pen Llyn and Deiniolen.
Two of the bigger and easiest accessed crags are described here and for more information on the rest of the circuit, please see the North Wales Bouldering guide.