Warcraft III: The Frozen Throne is a real-time strategy computer game developed for Microsoft Windows, Mac OS and Mac OS X by Blizzard Entertainment; it can also be played on Linux systems by using Wine or Cedega. It is the official expansion pack to Warcraft III: Reign of Chaos, requiring the ownership of Reign of Chaos to play. Released in stores worldwide in multiple languages beginning on July 1, 2003, it includes new units for each race, a new neutral race, four campaigns, five neutral heroes (an additional neutral hero was added April 2004 and two more were added in August 2004), the ability to build a shop and various other improvements such as the ability to queue upgrades. Sea units were reintroduced; they had been present in Warcraft II but were absent in Reign of Chaos. Blizzard Entertainment regularly releases patches for the game to fix bugs, add new features, and balance multiplayer play.