Complex sentence: Waller had been informed of their arrival, and he abandoned the siege and marched to occupy the highest point of Roundway Down, hoping to defeat Wilmot before Hopton could sally from Devizes against his rear.
Split into simpler sentences: Waller had been informed of their arrival, and he abandoned the siege and marched to occupy the highest point of Roundway Down. He hoped to defeat Wilmot before Hopton could sally from Devizes against his rear.