According to some authorities, Castile soap originated in the Kingdom of Castile, where it was made from olive oil only; importations of "Castile soap" through Antwerp appear in the London port books of 1567–68 (Dietz 1972), though the OED could find no reference to "Castile soap" earlier than 1616. In Castile, barilla (an impure form of sodium carbonate obtained from plant ashes) was boiled with locally available olive oil, instead of tallow. By adding brine to the boiled liquor, the soap was made to float to the surface, where it could be skimmed off by the soap-boiler, leaving the excess lye and impurities to settle out. This produced what was probably the first white hard soap, which hardened further as it was aged, without losing its whiteness, forming jabón de Castilla, which eventually became the generic name. To an apothecary it was known as sapo hispaniensis or sapo castilliensis
The fine ashes used in soap-making were so valuable that they were shipped to England from Massachusetts in the 17th century. The British Parliament decided to permit free importation of some products used in linen manufacture in 1803, when the war was creating scarcity: among the ingredients to come free of excise were "wood and weed-ashes, pot pearl ashes, barilla..."
Another theory is that modern-day castile soaps are conceptually similar to, and most likely derived from the so-called white soap that has traditionally been manufactured in Northern Italy since the early 1600s. From Venice, where the oldest white soap factories were established, olive oil-based soap reached the Greek island of Crete first, and Southern France (Marseille) at a later stage.