Complex sentence: However, the castellan Brian de Lisle refused to hand them over; although Lisle and Ferrers were both John's supporters, John gave Ferrers permission to use force to take the castles.
Split into simpler sentences: However, the castellan Brian de Lisle refused to hand them over. Although de Lisle and Ferrers were both King John's supporters, the king gave Ferrers permission to use force to re-take the castles.