Crystal Castles has nine levels with four castles each, and a tenth level which features a single castle — the clearing of which ends the game. Each of the 37 trimetric-projected castles consists of a maze of hallways filled with gems and bonus objects, and also includes stairs, elevators and tunnels that the player can use as shortcuts. The three-letter initials of the player with the highest score are used to form the first level’s castle structure. When all gems in a castle have been collected, the player moves on to the next castle. The player can also skip some castles and acquire additional lives and points by using secret warps activated by making Bentley Bear jump at special locations. A trackball and “jump” button are used for controlling Bentley Bear’s motions. Gems are collected by simply walking over them, and a bonus is given upon collection of the last gem. While collecting gems, there are a number of enemies that try to stop Bentley and/or collect the gems for themselves. With two exceptions, if touched by the enemies he will lose one of his lives. Any gems collected by the enemies also result in a lower obtainable score for that screen. Likewise, if the last available gem is collected by the enemy, the player also loses the last gem bonus. Enemies can be avoided by use of the maze and its constructs, or by making Bentley jump over opponents with the jump button, in some cases also allowing him to stun them. Some types of enemies will track Bentley’s movements in certain ways, while others move at random. If Bentley is touched, he “cries out” in a distinctive manner with the use of a cartoonish word balloons. If 3 or more lives remain, he says “BYE!”; if 2 lives still remain, the quotation is “OH NO!”; if 1 life is left, it is “OUCH!”; and finally, for the last lost life (which ends the game), he says “#?!”, so as to imitate an obscenity. At the beginning of every maze, gems are worth 1 point each; this value increases by 1 for every gem Bentley picks up, to a maximum of 99. Each maze also randomly includes a hat or honey pot, which serve the dual purpose of awarding points and providing Bentley with the ability to defeat specific enemies. The hat is worth 500 points and will make Bentley invulnerable for a few seconds and allow him to eliminate Berthilda the witch, who appears in the last maze of each level. The honey pot is worth 1,000 points, and picking it up can delay the landing of a swarm of bees. Other villains present in the game include: “Nasty Trees” which become more ornery as levels progress, a ghost that will usually appear in the Hidden Spiral levels, dancing skeletons, “Gem Eaters” who Bentley Bear can defeat if he catches them while eating a gem, and also the devilish “Crystal Balls” creatures that in later levels tend to follow Bentley Bear persistently as he collects gems. The Nasty Trees and Crystal Balls can also pick up gems. Crystal Castles contains two notable easter eggs. Jumping 100 times or more in the southeast corner of level 1?1 and clearing the maze of all gems will make ATARI appear on level 1?2.[2] On level 5?4, if the player kills Berthilda and goes to the corner of the area where she was and jumps, “FXL” will appear in the southeast corner of the screen. These are the initials for Franz X. Lanzinger, a designer of Crystal Castles.

Similar to Centipede, the object of the game is to destroy a millipede that advances downward from the top of the screen. The millipede travels horizontally until it either hits an obstacle or reaches the edge of the screen, after which it drops one row and reverses direction. Once it enters the player’s gray maneuvering area, it stays there and extra heads appear at intervals until both they and the millipede are destroyed. Shooting a body segment splits the millipede in two, with the rear portion sprouting its own head. A collision with any enemy costs the player one life. New enemies and gameplay elements are introduced in Millipede: Earwig: same as the scorpion in Centipede, making mushrooms poisonous so that the millipede will charge straight to the bottom of the screen after touching them. Bee: same as the flea in Centipede, dropping mushrooms in a vertical line and requiring two shots to kill. Spider: same behavior as in Centipede, bouncing irregularly across the player area and eating mushrooms. Multiple spiders can appear at the same time on higher levels. Inchworm: when hit, slows all enemies for a short period of time. Beetle: crawls around the player area for a while, then climbs up and leaves the screen, turning any mushrooms it touches into indestructible flowers. When hit, everything on the screen scrolls down one row. Dragonfly: drops mushrooms while zigzagging down, and can be destroyed with a single shot. Mosquito: bounces off the sides of the screen as it descends diagonally. When hit, everything on the screen scrolls up one row. DDT bomb (stationary): can be blown up with one shot, destroying all enemies and mushrooms within the blast radius. Whenever the mushrooms scroll down, a new bomb is added at the top of the screen. Up to four bombs can be in play at one time. The player scores points for shooting the bomb itself, as well as increased values for any enemies destroyed in the blast. All flowers and poisoned/partially destroyed mushrooms revert to normal, whole mushrooms and score points during the process when the player loses a life. At regular intervals during the game, the player will face a swarm of enemies (bees, dragonflies, etc.) instead of the usual millipede. Each enemy destroyed awards increasing points, up to a maximum of 1,000 points per enemy; this attack ends when either the entire swarm has passed or the player loses a life. Also, at intervals new mushrooms will grow on the field while others die off, in a pattern similar to Conway’s Game of Life. Players can also choose at the start of the game whether to play at an advanced level, starting with a score that is a multiple of the number of points needed to earn an extra life (by default, 15,000). The gameplay is generally much more advanced than it would be had the player started with a score of 0 and worked their way up to that point level. The maximum advanced level allowed is a function of the preceding player’s score, and games started at an advanced level where the player did not earn at least one extra life are not eligible for the high scoreboard.