1. Add an optional layer of difficulty to the game
2. Skulls do not appear on the map until assigned objectives are met
3. Kill a specific number of a certain unit type in each mission to earn a Skull
4. Skull location flashes on mini-map once objective is met
5. Watch the screen to track the location
6. Access the Skull via menu system
7. Activate to trigger special effect

Each mission begins with all Skulls deactivated. They may be activated at any point during the mission. Once activated, however, they may not be deactivated until the mission has been completed.

At the end of each level, you receive a cumulative score based on the primary and optional objectives that have been completed, combat bonus calculated from the ratio of enemies killed to allies lost, time bonus and Skulls bonus. Your score determines if you receive a gold, silver, bronze or tin award.