The word castile is mentioned once during Harper Lee's novel To Kill a Mockingbird. It can be found in Chapter 12 as part of Scout's narration of the rough bath given to her by Calpurnia prior to their visit to the First Purchase A.M.E. Church.
She made me soap all over twice, drew fresh water in the tub for each rinse; she stuck my head in the basin and washed it with Octagon soap and castile.
Octagon soap was a popular soap in America during the 1930s. It was made by Colgate and was known for its ability as a stain and grease remover. Not only was it used for bathing, but it was also used to clean stoves and woodwork. Castile is a specific type of soap, usually made with olive oil; originating in the 16th century (probably in Belgium or France), it was probably the first type of hard bar soap, used primarily as a body wash, shampoo and conditioner.