Homeworld 2 is a real-time strategy video game sequel to Homeworld, developed by Relic Entertainment and released in 2003 by now defunct publisher Sierra Entertainment. Its story concerns Hiigara's response to a new enemy called the Vaygr. Its gameplay takes a new direction with the enhancement of its graphics and audio. In contrast to the closely equivalent Kushan and Taiidan forces of the original game, Vaygr and Hiigaran spaceships differ significantly in design and application. Gameplay Homeworld 2 shares the movement system and three-dimensional play area of its predecessor. Units are moved three-dimensionally using a combination of mouse movements and key presses to modify the z-axis of the destination position. Gameplay in Homeworld 2 depends on so-called "Resource Units," (RUs) which are collected by harvester craft. RUs are the currency for both ship creation and technology research, placing limits on what combination of shipbuilding and new technology research can be carried out in a single game. The player's fleet is centered on the Mothership, the destruction of which results in an immediate Game Over in a single-player game, and is a critical loss in a multiplayer game. The Mothership is capable of constructing all but the very largest ships, which must be built using Shipyards (the game states these are built at orbital facilities and arrive via hyperspace). The Mothership can also build Carriers, which themselves can build any ship from the Fighter, Corvette and Frigate families. Although the Mothership is initially the center of new research, Carriers and Shipyards can build their own on-board Research Labs. All units in Homeworld 2 are starships of various classes, from small to large: Fighter, Corvette, Frigate and Capital Ship. Although the larger ships are more powerful and durable, they are also much slower and less maneuverable. Ships within each class specialize in certain areas and are vulnerable to certain other ships, although there may be ships in the same class that do not share the same vulnerability. Ion Cannon Frigates, for instance, are very vulnerable to fighters due to the unwieldy (albeit powerful) nature of their single weapon. Flak Frigates, however, are specifically intended for use against fighters, but lack the firepower to defend themselves against other frigates. Effective use of the relative strengths of each ship is essential for success. Homeworld 2 allows players to co-opt enemy ships using specialized frigates, much like the original game. These maneuver close to enemy