Complex sentence: Public access to the Park has been restricted since World War II when the Army erected a fence to guard against sabotage of the city's water supply, except for specially arranged biannual tours, attended by scores of visitors curious about the odd-looking structures.
Split into simpler sentences: Public access to the Park has been restricted since World War II when the Army erected a fence to guard against sabotage of the city's water supply. Specially arranged biannual tours are supported by scores of visitors curious about the odd-looking structures.