The develoment of the dock system and associated buildings began in 1793 when the Assheton-Smith family of Vaynol built a small quay on a marshy inlet to ship the output of its Dinorwic quarries. It stood immediately downstream of a corn mill worked by the tide, which had given the area its Welsh name - Y Felinheli - and displaced an earlier arrangement whereby slate was lightered into the Menai Straits from 500m to the south-west. It came to be extended in stages both up and downstream until 1844-5 when the 'cei mawr', a tidal dock, was completed. The walls were built of roughly-squared limestone laid without mortar and backed with rough-hewn slate slabs. Between 1897 and 1900 the sites of the earlier quays were rebuilt for non-tidal operations by Thomas Ayres Ltd., the last major investment in a Welsh port. The quays have since been re-developed for housing though structures from the slate-shipping era have survived.
The port complex includes the entire dock system (NPRN 302076), the dock offices (NPRN 302077) and the South Dock with its quay walls (NPRN 302078).
Source:
D.Gwyn, Welsh Slate: the Archaeology and History of an Industry (RCAHMW 2015), p.225-7.

D Leighton & S Garfi, RCAHMW, 19 June 2015