Gameplay
Door Door's blend of puzzle and platform game elements requires strategy, anticipation, and dexterity. Players control Chun, a small, spherical creature outfitted with a baseball cap. Chun is relentlessly pursued by a quartet of computer-controlled aliens traveling in unique deterministic paths. The most predictable aliens Namegon and Amechan follow Chun in the most direct path possible, Invekun deviates and follows roundabout paths using ladders, and Otapyon shadows Chun's jumps.
The player's objective is to trap the aliens behind sliding doors positioned throughout each level, courses composed of platforms conjoined by assorted ladders. To trap the aliens, players approach the door from the side its handle is on, open it by running across it, lure the advancing villains inside, and shut the door before they escape. Trapped doors cannot be opened again. Chun can jump to avoid the aliens, whose touch spells death. Bombs and nails, which sometimes appear on the screen, are also lethal. When the player dies (provided he has continues) he restarts the level with previously trapped aliens vanished, and all doors are accessible again.
The status bar running along the top of the playing screen gives the player's score, the high score, the level number, and the number of lives remaining. Points are awarded for trapping aliens behind doors (with extra points going to players who corral multiple aliens behind one door) and collecting confectionery that intermittently appear and disappear on the playing screen, which include a striped piece of candy, an ice pop, a lollipop, a bowl of ice cream, a slice of cake, and a Mahjong tile. Players begin with three lives. Scoring 10,000 points awards the player an extra life; scoring intervals of 20,000 points thereafter awards the player more lives. If the player loses all lives, the game is over, forcing the player to restart from the first level.
As players advance, the levels become more complicated, many requiring abstract strategies. The eighth level pits Chun against a lone Otapyon alien; the player's strategy requires purposely allowing the Otapyon to shadow his jumps with the intention of guiding the unsuspecting Otapyon across the level's myriad platforms and into the doorway.
If an alien remains with all doors closed, the player is placed in a no-win situation. The tenth level, a flat expanse with two Namegon aliens and one door, illustrates this dilemma. Because the level begins with them spaced far apart, it is initially impossible to guide both aliens in the single door without one escaping. Allowing one to escape, however, places it closer to its counterpart, in turn enabling the player to trap both. If the player traps only one, then he must sacrifice one life as the stage will be impossible to complete.
The 50th level requires such timing that failure at any part guarantees an impossible-to-win scenario. As in the previous case, the player's only option is to kamikaze to continue play. This is not a programming error, but another aspect of the game's difficulty level. This is a unique trait of Door Door as most games prevent such a scenario with a time limit (such as Super Mario Bros.) or physical restriction (such as Tetris). There is no ending in Door Door; after completing the final level, the game simply restarts at the first level, although the player keeps whose allocated score and is allowed to build upon it with successive plays through the game.
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