E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Ancient ships had a castle, as may be seen in the tapestry of the House of Lords, representing the Spanish Armada. The term forecastle means before the castle. The Romans called the castled ships navs turritæ.
That part of the upper deck forward of the foremast In merchant ships, the forward part of the vessel, under the deck, where the sailors live.Dana: Seamans; Manual, p. 96.