Complex sentence: The main defensive fort for New London, Fort Griswold, located across the Thames River in Groton, was well known to Arnold, who sold its secrets to the British fleet so that they could avoid its artillery fire.
Split into simpler sentences: The main defensive fort for New London was Fort Griswold, located across the Thames River in Groton. It was well known to Arnold, who sold its secrets to the British fleet so that they could avoid its artillery fire.