Complex sentence: The Castilian forces abandoned Santarém, Torres Vedras, Torres Novas, many other towns were delivered to John I by Portuguese nobles from the Castilian side and the stability of the Portuguese throne was permanently secured.
Split into simpler sentences: The Castilian forces abandoned Santarém, Torres Vedras and Torres Novas, and many other towns were delivered to John I by Portuguese nobles from the Castilian side. As a result, the stability of the Portuguese throne was permanently secured.