Complex sentence: He sent an army under Alonso Pérez de Guzmán to take Gibraltar, which he eventually did after a month-long siege, and Gibraltar was settled by the Castilians for the first time.
Split into simpler sentences: He sent an army under Alonso Pérez de Guzmán to take Gibraltar. De Guzmán succeeded after a month-long siege, and Gibraltar was settled by the Castilians for the first time.