Welcome to the Armory Exhibition! The top ten list is complete. The sponsorship program is open. The complexity is increasing. The first three floors pale in comparison to what Carl and Donut are now facing. Iron ball. An incredibly complex subway system built from the world's underground railway systems, combined and then linked into a knot. Up is down. Down up. Close is far. The cars are full of monsters, the train stations are unsafe, and the exit is always just a few stops away. But there is hope. This is the first time the crawlers are working together. Loot is better than ever. And the answer to all this may be hidden on the pages of a seemingly useless book. Welcome, crawlers! Welcome to the fourth floor of the Dungeon!