Shield-Wall
The Shield-Wall symbolizes the different ways that people can view life. London looks at existence like a battle, where the goal is to conquer and win. But the Anti-Traction League is more focused on survival and preservation than conquest—they do not want to risk their lives to gain power. This is reflected in their decision to build the wall rather than directly engage with cities like London. It represents this difference between the two groups: one that seeks immediate gratification and wants to take what they can while they can, and another that values long-term sustainability and peace over short-term gains. In addition, the wall also represents hope for both sides of this conflict. It suggests that there is still potential for peace even in a world full of warring cities. Since London fails in its attempt to breach it, there may still be some way for them to coexist with other settlements.
Hester’s Scar
Hester is an orphan whose parents were murdered by Valentine. Her scar represents the physical trauma of the attack and the emotional trauma of losing her parents. Even though it has left a permanent mark on her face, it has not broken her spirit or made her incapable of trust. Instead, she has channeled that pain into courage and resilience that allows her to fight against Valentine throughout the story. Hester's scar also serves as a signifier of how people may judge each other based solely on their outward appearance. Although many characters initially assume that Hester is dangerous because of the way she looks, they soon realize that there is much more to her than meets the eye. Tom especially notices this when he begins to understand Hester better and realizes that despite all she's been through, she still cares deeply for others and shows loyalty even in difficult situations.
Municipal Darwinism
Municipal Darwinism is an allegory for the culture of the Traction Cities, which are large cities that roam across the post-apocalyptic landscape. These cities consume smaller towns and villages to gain resources while leaving behind waste and destruction in their wake. In this way, they are akin to Charles Darwin’s theory of evolution – survival of the fittest – with larger cities devouring smaller ones. Tom and Hester learn about how traction cities evolved from small towns and villages into huge mechanized behemoths over time. The museum's curator explains that these settlements had to become mobile because otherwise they would have been absorbed by larger cities. They soon discover that most of these cities view themselves as superior to all others and will often attack weaker settlements without warning or mercy to gain resources. Municipal Darwinism also serves as an allegory for real-world politics and power struggles between nations and cultures—those who have more resources tend to use them to dominate weaker nations or cultures, often resulting in war or exploitation.
Technology
At its most basic level, technology is used to power the massive mobile cities that make up much of this world. These cities have been built from old machines scavenged before civilization's collapse. They use enormous traction engines to move around, powered by burning fossil fuels like oil, gas, and coal. Technology also plays an important role in everyday life for the citizens of these mobile cities. Tom relies heavily on various pieces of mechanical equipment throughout the novel including his trusty gunstick and later even a steam robot. These devices are all powered by advanced technologies such as clockwork or electro-magnetism which allow them to perform incredible feats such as shooting bullets with pin-point accuracy or even walking upright as a humanoid figure. Whatever is left of humanity in this world must either embrace technology or be crushed beneath its wheels.
Social Class
The mobile cities form a new society and culture divided into three distinct social classes: high, middle, and low. At the top of this new society are members of the High Council who run the cities from their airships. They wear fine clothes, eat expensive food, live in luxurious surroundings, and enjoy many privileges that no other citizens have access to. Next is the middle class which consists mainly of engineers who maintain the engines that drive each city forward. It is a relatively comfortable lifestyle with access to some luxuries but not nearly as privileged or influential as those at the top of society. Finally, the lower class consists mostly of scavengers living on what they can find in scrapyards or ruins left behind after battles with rival cities. This class lives hand-to-mouth without much hope for upward mobility since they lack any real resources to improve their situation within society.