stages in different groups to create themes. Early stages were designed to be relatively simple compared to later ones. The level of difficulty was designed to increase quickly so players would struggle to complete later stages. In retrospect, Jarvis attributes his and DeMar's average player skills to the game's balanced design. Though they made the game as difficult as they could, the high end of their skills ended up being a good challenge for expert players. The graphics were given a simple appearance to avoid a cluttered game screen, and object designs were made distinct from each other to avoid confusion. Black was chosen as the background color to help characters stand out and reduce clutter. Enemy designs Each enemy was designed to exhibit a unique behavior toward the character; random elements were programmed into the enemies' behaviors to make the game more interesting. The first two designed were the simplest: Electrodes and Grunts. Electrodes are stationary objects that are lethal to the in-game characters, and Grunts are simple robots that chase the protagonist by plotting the shortest path to him. Grunts were designed to overwhelm the player with large groups. While testing the game with the new control system and the two enemies, Jarvis and DeMar were impressed by the gameplay's excitement and fun. As a result, they began steadily increasing the number of on-screen enemies to over a hundred to see if more enemies would generate more enjoyment. Other enemies were created to add more variety. Large, indestructible Hulks, inspired by an enemy in Berzerk, were added to kill the humans on the stage. Though they cannot be destroyed, the developers decided to have the protagonist's projectiles slow the Hulk's movement as a way to help the player. Levitating Enforcers were added as enemies that could shoot back at the main character; Jarvis and DeMar liked the idea of a floating robot and felt it would be easier to animate. A projectile algorithm was devised for Enforcers to simulate enemy artificial intelligence. The developers felt a simple algorithm of shooting directly at the protagonist would be ineffective because the character's constant motion would always result in a miss. Random elements were added to make the projectile more unpredictable; the Enforcer aims at a random location in a ten-pixel radius around the character, and random acceleration curves the trajectory. To further differentiate Enforcers, Jarvis devised the Spheroid enemy as