Complex sentence: The castle of Burgos and the cities of Plasencia and Arévalo were controlled by the Estúñiga family, supporters of Juana, although the actual city of Burgos, namely, the Fernandez de Velasco family backed Isabella.
Split into simpler sentences: The castle of Burgos and the cities of Plasencia and Arévalo were controlled by the Estúñiga family, supporters of Juana. The city of Burgos, controlled by the Fernandez de Velasco family, backed Isabella.