In Civ III, unit promotions involved rising in level from green to veteran to elite and each level added a hit point and a small combat bonus. For Civ IV, we decided to build on this system and make promotions much more intricate and customizable by allowing players to choose from over 20 different bonus types (many with multiple levels, for a total of over 40 available promotions). The numbers and types of available bonuses differ per unit type and only land and sea military units can acquire them (air units are different).
Here’s how it works: Units can gain experience points in several ways. Successful combat is the primary means (note: you get more points from attacking than you do from defending) and the units must be victorious in a unit-to-unit combat, therefore there are no XPs (experience points) from bombarding fortifications or if the opposing unit withdraws. Units can also gain experience from a Barracks (land units), a Drydock (sea units) or a goody hut. In addition, some wonders, civic choices, and leader traits can give units extra experience or even certain promotions. As you might expect, the first promotion comes rather quickly, while each subsequent promotion requires more XP.
The promotions take on many different forms. Some are simple like Combat (5 levels), which gives progressive strength bonuses with some extra healing at the top levels, or City Raider/City Garrison which gives the unit bonuses only when attacking/defending a city. Others give bonuses against a certain type of unit. For instance, Cover gives a bonus against Archery units and Formation gives a similar bonus against Mounted units, while Shock does the same against melee units. Some promotions give bonuses based on the type of terrain in which the unit resides. If your opponent has a lot of woods or jungle, you might choose Woodsman, or if your opponent is surrounded by hills you might choose Guerilla.
All in all, the new promotion system in Civ IV offers players a plethora of interesting new decisions, excellent customization options, and an extra layer of strategy. That’s how we spell fun at Firaxis Games.