Reinforcements, which can be called in any time to designated areas when allowed. In land battles, a player can field more units at a time if they capture Reinforcement Points. The storyline campaign follows a semi-open linear path, where the given side must complete sequential mission objectives (such as stealing X-wing prototypes). Campaign missions build up to the plot of Star Wars and eventually, the Battle of Yavin. The campaign is very similar to Galactic Conquest, although the Galactic Map is gradually opened for play as the player completes scripted events and completes missions. Skirmish mode is the familiar, classic RTS mode. Skirmishes come in two types: land and space. In land skirmishes, all players start with bases and must research upgrades and capture reinforcement points while fighting to destroy the enemy's base or command center. In space skirmish mode, all players have a space station that can be upgraded. The player buys land or space units to attack the enemy with until the enemy base/space station is destroyed or the enemy forces are destroyed. The player can advance in technology levels to access more powerful ships as the player gains more money. The player can build more buildings and defenses, and the space station can also be upgraded to better defend itself without requiring the player to leave ships behind to defend it. Skirmish battles require the least time investment, so it is better suited toward casual players. Maps can support two, three, or four players, and the difficulty can be switched between easy, medium or hard. All modes are played in real time. Days on the galactic map pass continually. When a battle occurs, galactic time is paused while the battle is waged. After the battle, the galactic day timer resumes. When playing against the computer, the player can accelerate time or pause time on both the galactic and skirmish maps. All modes also make use of important Star Wars characters, such as Darth Vader, as Heroes. Each Hero is a single, moderately powerful unit with special abilities. For example, Obi-Wan Kenobi, Emperor Palpatine, and Darth Vader can use the Force. Some characters can be used in both land and space battles, (e.g. Darth Vader becomes a special TIE fighter unit in space), while others can be used only on land or only in space (a land-only Hero will remain on board a landing craft or command ship