The game starts on a world map with only one accessible Carrier level (read below). Beating this level opens the "Easy" set of Carrier levels. When these are beaten, the next set of Carrier levels is opened. Bonus levels are earned by finding and activating communication devices hidden in some levels. And so, the game expands from a single Carrier level to many different levels of two types (Carrier and Bonus). Each level is initially marked with a shadow, but this can be filled with a medal by fulfilling the secondary requirements of a Carrier mission (by destroying buildings, freeing survivors, and collecting RDUs), or by getting a good enough time in a Bonus mission. Each level is also circled in a green or red outline. A green outline means that there is at least one comm. device in the level that has not been found, whereas a red outline means that all of the satellites in the level have been found.