Jabon de Castilla or Castile soap originated in Spain, and was named after a region of modern day Spain known then as the Kingdom of Castilla due to the amount of castles that had been built there to reconquer the area back from the Moors.
At that time soap making in northern Europe focused on the use of animal fats, whereas in Spain the abundance of olive oil lead to a pure olive oil soap that became known as Castile soap. Due to its incredible moisturizing properties, this soap became a luxury commodity in the rest of Europe, being a favorite amongst royalty and noblemen. The infamous Louis the fourteenth of France became so hooked that he ordered French soapmakers to make the switch to olive oil, and this tradition continues to this day- commonly known as Savon de Marseille.
Many soaps are sold with the castile label even though some have very little olive oil in their formulations, a true castile soap should have 100% olive oil, and preferably extra virgin olive oil. A pure olive oil soap is unlike other soaps in that it doesnt produce an abundant fluffy lather, but rather a creamy, small bubbled lather that leaves the skin extremely moisturised and is suitable for extremely sensitive skin, babies, and small children. Olive oil is naturally rich in Oleic acid, containing between 60-80% which means that an olive oil is a natural moisturiser helping to attact moisture from the surrounding air inturn keeping skin supple and soft. The other extremely important ingredient in Olive oil is Squalene, and Olive squalene is close in chemical structure to the skins own squalene which means that it can penetrate the skin effectively and inturn this helps the skin to stay moisturized and helps it to naturally regenerate, even helping to heal damaged skin. To find out more about how the my castile soap is made there are more details in this former post
All this makes choosing a Castile soap for your daily hygiene routine a good choice expecially if you have dry and sensitive skin. Or if you are looking for a safe natural soap for newborns or small children. Just remember not to expect the same sort of lathering properties in other soaps, even natural ones.