Castile soap is an amazingly versatile vegetable-based soap that's made free of animal fats and synthetic ingredients. This natural, non-toxic, biodegradable soap is available in bar or liquid form. ... It gets its name from the Castile region of Spain Castile soap is an exciting product since it’s not only environmentally friendly but also incredibly useful and effective for certain purposes. You can use it on your body, clean your house, and even on pets. You may find that a bottle of castile soap conveniently replaces a plethora of other products in your home.