By the MiddleAges, Welsh slate was
being shipped all around the British Isles, and
maybe even abroad. When Chester castle
was renovated in 1358 under the supervision
of Edward the Black Prince. 21,000 slates
were shipped from Wales to cover its roof.
Many other castles and large houses used
slate from the Welsh quarries at Cilgwyn and
Penrhyn. Further north in Scotland, a thick,
more durable kind of slate was used on the
roofs of medieval houses in Edinburgh.