On the river Conwy, cargoes of slate to a local point of use are recorded as early as 1710 and continued until 1879, shipping the output of the small quarries above Dolgarrog and Trefriw and in the Lledr, Llugwy and Machno valleys. Although the principal quay was at Trefriw, slate was also shipped at Conwy itself.
The port of Conwy in late medieval times had not been of great economic importance, and its principal function was the import of essential goods, and its use as a fishing port. This basic use continued until the nineteenth century, when the expansion of the fishing industry combined with the transportation of products associated with the industrialisation of North Wales, particularly metal ores and slates, led to an increase in the demand for port facilities. In addition, a quay higher up the river had become unusable following changes in the course of the river, possibly caused by the construction of the embankment and bridge. The flat-bottomed boats bringing slates and metal ores down-river from Trefriw were therefore now unloaded at Conwy quay.
Sources:
D.Gwyn, Welsh Slate: the Archaeology and History of an Industry (RCAHMW 2015), p.221.
Gwynedd Archaeological Trust, Conwy Quay: Archaeological Assessment, Report no.273 (1997).

David Leighton, RCAHMW, 18 June 2015