The Extra Stage (エキストラステージ Ekisutora Sutēji ) is a hidden stage hidden inside various Drillimation games, with it mainly appearing in the Touhou Project series since Touhou 2: Story of Eastern Wonderland. As the name suggests, they are really difficult to complete and requires multiple credits to play. There is generally one extra stage in almost all Drillimation games, while some games have more than one.
Touhou 7: Perfect Cherry Blossom was the first game to have more than one extra stage, being the Phantasm stage. In order to gain access to it, the player must beat the game at least once, and the player can only choose the character and weapon type based on what they beat the game with. In the arcade versions of Drillimation games, the extra stage is unlocked by default although it is hidden in the game's difficulty selection screen, as the player needs to put in a simple code to unlock it.
Touhou Project series
In the Touhou Series, there has been one extra stage since Touhou 2: Story of Eastern Wonderland. According to Nintendo Power, the extra stages of Touhou 6: The Embodiment of Scarlet Devil and Touhou 7: Perfect Cherry Blossom are considered to be the toughest to beat, and the only way to get through them easily is to cheat.
Mr. Driller series
Extra stages have appeared in the Mr. Driller series since Mr. Driller X. Every world has one extra stage, and the only way to gain access to them is to complete the game first and collect all of the Star Coins in that world to unlock it.