In September 1781, British troops under Benedict Arnold raided and burned New London in the Battle of Groton Heights with the objective of ending the harassment at sea. Fort Griswold had a key strategic position above the Thames River, but the British knew the fort's inner workings thanks to the turncoat (Arnold) who gave numerous secrets of American defenses to the British forces he commanded. Arnold, knowing Griswold's layout and precise position, approached the harbor from such an angle that Griswold's gun positions could never draw an effective shot on the British fleet. Arnold's troops eventually made landfall, and the fort's garrison fought back. Artillery barrages and musket fire brought (relatively) heavy casualties to each side, the fighting continuing even past commands to stop.