The shot of a city which opens the story is actually of Rouen, France. The spire near the center is Rouen Cathedral, which for four years in the 1800s was the tallest building in the world. The same shot was also used to represent other fictional nations in Robot (1969) and Time Bomb (1969)
Set of party scene has been used in 4 previous episodes of season 4: 4.14, 4.15, 4.16, and 4.25.
The framed photo Peter Malik with his mother and father is actually young Leonard Nimoy.
Paris (Leonard Nimoy) is posing as a 25 year old "Peter Malik" (dialogue establishes Malik died 20 years before at age 5). Nimoy was in fact 39 at the time.
Barney's motorcycle is a 1969 Honda CL-350 (350cc, twin cylinder).