A large portion of the series' success lies in the creation of a con lengthy design process with the release of each game. The overall design of each faction was slowly developed before the release of the first game in the series, Halo: Combat Evolved, and continually refined in later games' development. Reception of Halos factions has generally been positive. The Covenant were praised by reviewers as exciting and challenging enemies, and later games featuring the Forerunner Prometheans as enemies were criticized as not living up to the same standard. Characters, weapons, and vehicles of all the factions have been released as toys or promotional materials.
