The symbolism of the plane
For the characters in the film, the plane is a new invention never before used in war (or for that matter, in life). In that sense, the planes in the film are symbolic of a new way of fighting war and killing people, as well as an all-seeing eye.
The symbolism of the "grand illusion"
The title of the film, The Grand Illusion, is a powerful symbol of how the upper class (or aristocracy) think they are often better than and too good to be involved in war (in the case of the film, WWI).
The symbolism of Germany
In the film, Germany, which started World War I, is symbolic of profound aggression, violence, and ability to do harm. They are, in other words, a powerful symbol of negativity.
The symbolism of the castle prison
The castle prison is a powerful symbol of the way that the rich often sit in the large properties and look down upon lower classes. Those properties are, in other words, their so-called "ivory tower."
The allegory for the futility of the aristocracy
The film is an allegory for the futility of the aristocracy. During the year the film takes place, the aristocracy across Europe (especially in France and Germany) starts to crumble. That, the film argues, is a good thing. The film doesn't see any value in the aristocracy.
