Expansion 0 is the base game of Gatekeepers the Game, that is why it is called Expansion 0 and not Expansion 1. It is not long, but introduces much content for later expansions (Domains, gatekeepers, The Player, the fire sword, The Player's guide, Domains, The Gate, currency, various social simulation components, and more) and includes a gatekeeper battle. The battle system was designed to be fun, addicting, very hard, and engaging; and is a central part of the game.