When C-Plus thinks about the Roman Empire being attacked by the Gaulish, he says, the back of his helmet looking like Darth Vader's, "When the Gaulish attack, the empire strikes back." The first notes of John Williams' "Imperial March" are playing in the background.
There is a scene in the movie where a black centurion chokes somebody, like Darth Vader did on Tantive IV, and says, "You won't fail me again." When the centurion throws the person away, there is a shot of his head and shoulders, almost identical to Vader's, and the "Imperial March" plays in the background. Then he says, "The Empire Strikes Back!"