Batman: Arkham Underworld is a game based on the Arkham series where the player needs to build and maintain a criminal empire in Gotham City, recruiting many villains from the DC Comics universe. The story starts nine year before the actual game when Catwoman helps Riddler escape from Arkham Asylum. From that moment mob boss Carmine Falcone is targeted right away to take over his businesses and territory, expanding gradually. The player also builds a base, using the main cash currency to build structures such as vaults to store additional cash, corridors, training rooms where thugs are upgraded, and a lab where new villain abilities can be researched. There also needs to be entertainment to keep the thugs happy. Catwoman is the main character who guides the player through the game.
Thugs, guided by villains that act as powerful heroes and have special abilities with a cooldown time, need to be controlled to complete various missions with squad management and real-time strategy gameplay. Those missions include offensive ones as well as defensive quests where a base need to be defended, for instance by setting up sentries as guards. Units can be positioned and moved, but then act on their own to engage and defend. Enemy strength for a unit is shown through a number and each unit has an individual health bar. Players can set a waypoint and thugs will then fight a way to that location on their own seeking the shortest path. Locations can often be scouted before engaging, for instance to find the optimal entry point. Players can recruit additional thugs and spend the currency at the black market. There are business items that generate cash over time and luxury items that generate respect. Completing missions is also rewarded with skulls, representing experience for the global player level. Super villains are unlocked by completing specific missions. They include Riddler, Harley Quinn, Bane, Scarecrow, Freeze, Killer Croc and more.
The game is free-to-play. Diamonds are the premium currency, mainly used to speed up actions that generally have waiting times to complete. There is a multiplayer component as raids integrated into the story. Investments into defenses for the hideout are also needed to protect against incoming raids. Despite the title, Batman is not part of the game.