(Northumberland) was once called Moncaster, from the monks who settled there in Anglo-Saxon times; it was called Newcastle from the castle built there by Robert, son of the Conqueror, in 1080, to defend the neighbourhood from the Scots.
Newcastle (Staffordshire) is so called from the new castle built to supply the place of an older one which stood at Chesterton-under-Line, about two miles distant.
Carry coals to Newcastle. A work of supererogation, Newcastle being the great seat of coals. The Latins have “Aquam mari infundere” (“To pour water into the sea”); “Sidera caelo addere” (“To add stars to the sky”); “Noctuas Athenas” (“To carry owls to Athens,” which abounds in them).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894