Complex sentence: Thevar took this torturous route instead of the highway to Tinnevelly to conceal his troop movement and surprise the enemy, basing his plan on a spy's report that Mudali had camped his bulk of force some 20 miles away where he anticipated Thevar's attack.
Split into simpler sentences: Thevar took this torturous route instead of highway to Tinnevelly so as to conceal his troop movement and surprise the enemy. Also the whole plan was based on spy report to Thevar that Mudali had camped his bulk of force some 20 miles away where he anticipated Thevar's attack.