Complex sentence: The battery has few cannons to the outside, only the necessary to support the defense of San Fernando but its use was to attract the sieging forces to enter in a tunnel that appear as accidentally opened 500 meters away from the fort.
Split into simpler sentences: The battery has few cannons to the outside, only those necessary to support the defense of San Fernando. Its use was to attract the sieging forces to enter a tunnel that appears to be accidentally opened 500 meters away from the fort.