Could this be the 2009 book The Roar by Emma Clayton?
In the future, a plague spreads through all the world's animals. To survive, the humans create a northern wall through Canada, England, etc. Every living thing south of the wall is destroyed while all the people in the world move north of the wall into multi-leveled cities with no plants or animals. Their food is produced from chemicals and mold.
Because of the lack of space, it's 30 years before anyone is allowed to have children. 2 of these children are fraternal twins Ellie and Mika. When Ellie disappears, Mika's parents believe that she's dead. But the strong telepathic bond Mika shares with his twin leads him to believe that she's alive, and he'll do anything to find her again. This leads him to follow along with what the government demands of him even when he senses there's some secret the government is trying to hide from everyone. This includes joining his classmates in learning war games on simulators and competing in competitions that can make his family very rich.
In another review:
The world behind The Wall was divided into two; the rich live in the upper level known as The Golden Turrets while the poor live on the lower level, The Shadows.