The Nomad Camps, later styled as The Xiongnu Empire, are the de facto enemy for the vast majority of the campaign in Emperor: Rise of the Middle Kingdom.
The Xiongnu Empire becomes an early problem for China and are the main enemy that needs to be watched for most of the game. Until the Han Dynasty, they are styled as The Nomad Camps.
Their first appearance is in Baoji. In this mission, they are peaceful and offer Jade for which Carved Jade can be made. It may be difficult to open trade with them, as they purchase goods that must be imported to the city.
The Xiongnu do not pose a major military threat during most of the Zhou Dynasty, though their threat looms over many of the missions. They invade Loyi, after having stormed through a desert outpost and destroyed Hao, the previous capital city, but their invasion can be quelled by giving into their demands.
Towards the end of the Zhou Dynasty, their role is diminished significantly due to the Warring States Period.
The Xiongnu's presence in the Qin Dynasty is significant. Although they will not invade during the early missions of the new dynasty, they do invade in the later missions of Badaling and Xianyang. They can invade during the construction of Xianyang, but only if the player takes too long to complete the mission.
Like the Zhou Dynasty, the Xiongnu only threaten China as a distant empire. To reference their now-powerful tribes, they rename themselves as The Xiongnu Empire, but don't pose much of a threat to the country. They can invade Loulan if their demands are ignored (which results in mission failure, due to mission restrictions), but are not scripted to invade any missions.
After the completion of the Han Dynasty, they are removed from the map and are replaced by various other empires for the remainder of the game.