The core team of developers was composed of 30 people, but counting testers and other outside contributors more than 100 people had worked on the game. Only three months were spent on the concept stage of development, but would spend the next two years on the prototype cycle of development. Numerous influences on the game were cited. During the beginning of the project the developers looked toward "Tom Clancy's Rainbow Six" and "Counter-Strike", stating that they wished to reach a level of intensity that fell somewhere between these two games. They also wished to implement a strategic element to the game, which was influenced by "X-COM", where a team-based system was used to overcome obstacles. "Halo" also influenced the game, but according to the developers only to a minimal extent, mostly in terms of interface design and combat balancing. Some aspects were admitted to being similar to "Halo" for the sake of making the game accessible to those who had played "Halo" before (the controls are, for example, almost identical).