Buildings and knowledge are required to acquire the ability to produce various troops. There are also a few special units, including elephants, desert raiders, and mounted bowmen. Each unit is rated by morale, health, and quality. Morale is affected by fortunes in battle, the empire's condition, recent battle losses, and the unit's health. Health is affected mainly by food supply and battle losses.
Quality is affected by a city's level of advancement and battle experience.
Additionally, when units and cities are exposed to each other, they affect each others' health and morale. It's hard to determine what, if any, advantages, disadvantages, or special abilities various units have. There are no hard numbers on attack strength, defense strength, fortification effects, terrain effects, and how exactly health, morale, and unit quality affect the final battle results. You have to play the game numerous times before you get a mental picture of some sort of comparison chart. Because of this, it is difficult to figure out the best battleplan and production plan for your empire.