In games with linear progression, levels are areas of a larger world, such as Green Hill Zone. Games may also feature interconnected levels, representing locations. Although the challenge in a game is often to defeat some sort of character, levels are sometimes designed with a movement challenge, such as a jumping puzzle, a form of obstacle course. Players must judge the distance between platforms or ledges and safely jump between them to reach the next area. These puzzles can slow the momentum down for players of fast action games; the first "Half-Life"s chapter, "Interloper", featured multiple moving platforms high in the air with enemies firing at the player from all sides.