Castile soap is quite interesting... It comes from the Castile region of Spain and is known for being made from 100% olive oil, rather than made with animal fats, like traditional soaps of the time. Olive oil is a mild, light-colored oil pressed from olives. (Read more about olive oil here.)
One of the more interesting facts about castile soap is the cure time... Most cold process soap is ready to go after curing for 4-6 weeks, but castile soap gets better as it ages! Some folks use castile soap as early as 4 months into its cure, some claim that 6 months is prime, and more still say that castile soap is best after curing for a full year! Wow!!