There are a lot of soaps available that are labelled "Castile". But what makes castile soap truly a castile?
Castile soap is said to have originated in the Castile region of Spain in the 1500s. It's likely that castile soap has origins going back to the famous Aleppo soaps made in the Levant region since at least the time of the Crusades and probably much earlier.
A traditionally made castile has only three ingredients - olive oil, water and lye (also known as sodium hydroxide) - and is usually left to cure for several months. It is hard, creamy white soap that has less lather than you may be used to, but which is very, very mild.
Modern, less traditional, castile soaps may have coconut oil added for bubbly lather, shea butter to moisturise, essential oils for fragrance.
And the answer to the question "What is a Castile soap?" Castile is an olive oil based soap, but the variations are only limited by the soap-maker's skill and imagination.
Alba Rosa Classic Castile Soap is available here.