In Emperor, you'll encounter 7 different enemies. Those are the Chinese, Xiongnu, Korean, Viet, Tibetan, Khitan and Mongol.

Every enemy has certain unit types: for example, some of them don't have cavalry, and most of them don't have catapults. In the following table you'll see how many percent of each troop type is in an enemy army when they invade your city:

Enemy Infantry Crossbow Cavalry Catapults
Xiongnu 30% - 70% -
Korean 70% 30% - -
Viet 50% 50% - -
Tibetan 80% - 20% -
Khitan 30% - 60% 10%
Mongol 20% 10% 50% 20%

Chinese armies consist of different forces, depending on the time period:

Chinese Infantry Crossbow Cavalry Chariot Catapults
before 1200 BCE 100% - - - -
1200 BCE - 500 BCE 40% - - 60% -
500 BCE - 350 BCE 30% - 10% 45% 15%
350 BCE - 200 BCE 35% 20% 15% 15% 15%
after 200 BCE 40% 25% 20% - 15%

Historical information

Information about the different soldiers and enemies from BreakAway:

Chinese soldiers

The earliest-available and most basic type of military unit is the infantryman. Once fully manned, an infantry encampment holds 16 halberd-armed, heavily armored soldiers. An infantryman can be trained rather quickly, and requires one load of weapons. Additionally, all "foot" infantry soldiers (enemy & friendly alike) know how to use a battering ram and are equally well versed in "torching" buildings.
Chariots become available toward the end of the Shang Dynasty, about 1200 BCE. Four chariots comprise a full company, and each requires one load of weapons and two loads of wood to build. Being horse-drawn, a chariot is much faster than foot infantry, and, being bow-armed, a chariot can inflict damage from a distance.
The Zhou Dynasty (1045 - 220 BCE), part of which was also known as the Warring States period, was a time of frequent conflict, and also many military advances. During this time a favorite Chinese weapon was developed: the crossbow. Each trained crossbowman comes at the cost of one load of weapons and a half-load of wood, but the ability to stand back and inflict deadly damage should not be underestimated. Keep a bucket brigade of inspectors at the ready: when attacking a city, archers are prone to fire flaming arrows!
Siege weapons of various types were also developed during the time of the Zhou. Four catapults comprise a full fort (each requires two loads of weapons and four loads of wood). A catapult's slow speed and low rate of fire is partly compensated by the large wallop of its projectile. Naturally, these boulder-flingers can be very useful for reducing the defenses of an enemy city and flattening buildings. In fact, when dispatched to attack a fortified city, your expeditionary force should not leave home without some in tow. Like all missile weapons in Emperor, a catapult projectile might "stray", thereby missing its intended target.
Plagued by seemingly incessant raids by skillful, bow-armed nomadic cavalry, the Chinese eventually developed their own cavalry formations, albeit more heavily armored and mounted on larger "western" horses. By the time bow-armed cavalry came into vogue in the late Zhou, war chariots had already fallen out of favor. Eight troopers comprise a cavalry squadron, and each requires one load of weapons. However, the required training time is considerably longer than an infantryman. Beware: like foot archers, cavalry have been known to loose flaming arrows at buildings.

Foreign enemies

These fierce horsemen from the northern plains first appeared during the Shang Dynasty, and were a long-time nemesis of the early Chinese. The typical Xiongnu warrior rides a small steppe pony and is an excellent archer, having been trained with the weapon since childhood. Xiongnu infantry (think of them as "dismounted cavalry") are also available, thus rendering your city's walls and gates vulnerable to assault. Xiongnu can also be used to represent other nomadic horsemen the Chinese clashed with, such as the Turks during the Sui Dynasty.
The Chinese established settlements in Koguryo (modern day North Korea), and clashed with native forces there. Korean military units have moderate armor, and consist of halberd-armed infantry and foot archers armed with a composite bow.
During the Qin and Han Dynasties the Chinese moved southwards and established settlements in Annam (modern day southern China and northern Vietnam), and battled with the native Viet population. The lightly armored Viet troops boast sword-armed infantry and foot archers wielding a composite bow.
The Tibetans, to the west and southwest of China, were first encountered during the Tang Dynasty. The heavily armored Tibetan military units consist of halberd-armed infantry and deadly bow-armed cavalry.
During the Northern Song bands of fierce Khitan warriors invaded China from the north, conquering several cities. The typical Khitan trooper is a mounted warrior armed with composite bow. The Khitan also have infantry and some siege weapons.
During the latter part of the Northern Song Dynasty the game's most fearsome enemy, the Mongols, will be encountered. The most common Mongol unit is mounted on a sturdy Mongolian pony and armed with a bow for ranged fire and short sword for melee. The Mongol army also boasts siege weapons, sword-armed infantry, and foot archers (the latter two representing dismounted cavalry) so that they can better assault and plunder your city.