destroyed. As aliens are defeated, their movement and the game's music both speed up. Defeating all the aliens brings another wave which starts lower, a loop which can continue endlessly. A special "mystery ship" will occasionally move across the top of the screen and award bonus points if destroyed. Development Space Invaders was developed by Japanese designer Tomohiro Nishikado, who spent a year designing the game and developing the necessary hardware to produce it. The game was a response to Atari's arcade game Breakout (1976). Nishikado wanted to adapt the same sense of achievement and tension from destroying targets one at a time, combining it with elements of target shooting games. The game uses a similar layout to that of Breakout but with different game mechanics; rather than bounce a ball to attack static objects, players are given the ability to fire projectiles at moving enemies. Nishikado added several interactive elements that he found lacking in earlier video games, such as the ability for enemies to react to the player's movement and fire back, and a game over triggered by the enemies killing the player (either by getting hit or enemies reaching the bottom of the screen) rather than simply a timer running out. He replaced the timer, typical of arcade games at the time, with descending aliens who effectively served a similar function, where the closer they came, the less time the player had left. Early enemy designs for the game included tanks, combat planes, and battleships. Nishikado, however, was not satisfied with the enemy movements; technical limitations made it difficult to simulate flying. Humans would have been easier to simulate, but the designer considered shooting them immoral. After the release of the 1974 anime Space Battleship Yamato in Japan, and seeing a magazine feature about Star Wars (1977), he thought of using a space theme. Nishikado drew inspiration for the aliens from a novel by H. G. Wells, The War of the Worlds, and created initial bitmap images after the octopus-like aliens. Other alien designs were modeled after squids and crabs. The game was originally titled Space Monsters after a popular song in Japan at the time, "Monster", but was changed to Space Invaders by the designer's superiors. Hardware Nishikado designed his own custom hardware and development tools for the game. The game uses an Intel 8080 central processing unit (CPU), displays raster graphics on a CRT monitor