The game is partly inspired by role-playing games that deal with secrets and betrayal, such as 'Werewolves of Miller's Hollow'. All classmates play one game together, as inhabitants of the fictional fortified city of Thunderford. Some players, however, are secretly betrayers and have to try to let the defence of the city fail, so that the Spanish army can invade.
The game is designed to trigger the player's sense of identity and desire to be part of a group, a psychological development that occurs often in teenagers around the age of 12 or 13 years. Players get assigned a particular role that is assigned to them, based on a small personality test that they perform as preparation to the game.
To further strengthen their new role and identity within the game, players get their own house that they can decorate and build up as much as they want (as long as they have enough money).
Some players are assigned the role of fortification master. They get to decide how to build up the outer defences of the city. This part was kept as close as possible to the actual historical defences of Dutch fortified cities in the 16th century. The fortification masters can only build up defences with money and information that they receive from the other players.
The game is played in class over the course of one week. In order to keep things exciting, every night the Spanish army tries to invade the city. If they succeed, actual houses in the city that belong to the players can be burned down. This gives the players an incentive to connect their own self interest (keeping their house intact) to the common good (the defence of the city).