Feudal France
The setting of the story, feudal France, is the most dominant concrete imagery in the novel. The important part of this imagery however, is not the concrete aspect of it (in many ways, it just looks like good old France), but rather, the abstract quality of it. It looks like real life on planet earth. To the young history students who are trapped backward in time, that is the most sublime aspect of all—it is precisely as their present time might feel, except different.
Viking imagery
The Viking imagery is a wake-up call to the trio. They all thought Vikings were a novelty of time, like an aesthetic, but then, suddenly, they are faced with real Vikings, and the prospect of violence becomes a real threat. These are not just friendly farmers on the move—they are foreign warlords who have come to overthrow Paris, and they have one thing on their mind: victory. Their introduction to the story gives the story a tense feeling, because the trio knows enough about Vikings to fear them.
The imagery of war
When the Vikings mention that they are determined to bring war to France, that is unwelcome news to the young trio. What used to be merely another fact to memorize has now become a real life threat. No longer is war just another thing to learn about in class—suddenly, it is presented to them (and the reader) through real imagery. If they do battle, the children may or may not survive. This isn't like the modern world. Violence is likely, and death is a real threat.
History as lived time
The difference between this story and real life is that this story places the students in history so that they have to experience the past as the present, creating this abstract imagery where history is lived in as real time. The imagery illuminates the reality of the past, and it underscores the value of human life in the past. It isn't just "history." It is "human history," and that means the children are able to empathize with these people and become interwoven in their communities.