Why did Vienna become a centre of espionage during the Cold War?

Following World War II, Vienna became a global centre of espionage primarily due to the city's division into zones of occupation by the Allied powers—the United States, the United Kingdom, the Soviet Union, and France.1

This unique administrative structure, which lasted for a decade, created a volatile environment where intelligence agencies from opposing sides of the Cold War operated in close proximity. The city's first district, which housed the majority of government offices, was designated as an international zone and was managed by each of the four occupying powers on a rotating basis, further facilitating the intense intelligence activities of the era.1