What major challenges does the protagonist face?

In The Dungeon Anarchist’s Cookbook, the protagonists Carl and Donut face major challenges navigating the Iron Tangle, a complex and dangerous subway system formed by intertwining the world's subterranean railways into a confusing knot. The environment is disorienting—up is down, close is far—and filled with monsters and unsafe stations. The difficulty escalates significantly compared to earlier floors, requiring the protagonists to work with other crawlers and rely on better loot and clues from a seemingly-useless book to find a way out.