Complex sentence: In 1580, the castle was besieged by the forces of Guelders, but it was defended bravely by the lord of the castle, Marten Schenk, when the Duke of Parma sent cavalry, the sieging army had to retreat.
Split into simpler sentences: In 1580, the castle was besieged by the forces of Guelders, but it was defended bravely by the lord of the castle, Marten Schenk. When the Duke of Parma sent cavalry, the besieging army had to retreat.