Proceed to the Civ4 Downloads Database
Enter the Civ4 Creation & Customization Forum
Below, you can read an overview about installing, downloading, and creating mods.
Customizing Civilization IV
Types of Mods
There is a wide variety of user-created modifications available, briefly explained as follows:
Modpacks: These generally bundle multiple additions together from the following categories, often adding new graphics as well as changing the gameplay. Some focus on a particular timeperiod (e.g. Ancient Rome, the Industrial Revolution), others on the entire length of history (from Stone Age to Future Age), while others still cover a particular theme (fantasy, sci-fi, etc).
Mod components: Less comprehensive that modpacks, these usually contain simple interface or gameplay changes and do not alter the core game as much. Examples include components that keep track of units’ wins and losses and others that add features like immigration, civil wars, and more.
Graphic mods, new units, and new civilizations: This type of content usually modifies the interface or graphics of the game, leaving the mechanics unchanged. Popular mods include things that alter the terrain to make it more Earth-like and ones that create a more user-friendly interface.
Scenarios, maps, and map scripts: Maps are, unsurprisingly, files that allow you to play a game of Civ4 in a premade setting. The most popular ones are Earth maps that place civilizations in historically-accurate locations and mimic reality. Scenarios are like maps, except that they contain preplaced units and cities, may have mod-like rule changes and custom content, and take place over a particular period of time. Map scripts, like the standard map scripts included with the game (Continents, Archipelago, etc.) allow for unique, randomly-generated maps.
Utilities:Â These are out-of-game tools that make modding or playing the game easier, simplifying the process of, say, installing mods or creating map files.
Keep in mind when downloading mods (explained in the next section) the amount to which you are willing to change the game mechanics. While things like maps and map scripts will not alter the gameplay, modpacks and scenarios can change the game rules dramatically. Experimentation to determine your personal preferences is best!
Downloading Files
All types of mods and content can be downloaded from the Civilization IV File Database, which organizes files by type (scenario, map, mod, etc). Almost all of the files there are ZIPs, a file type that can be opened using a program like 7-Zip, which is free. After finding a file or mod that you would like to use, simply download the ZIP or RAR file to your Desktop. You can then open it to extract the mod’s files from the compressed folder.
Note that mods for Civ4 (without expansions) are not compatible with Warlords (the first expansion) or Beyond the Sword (the second expansion). In fact, mods are not backwards- or forwards-compatible, though they can be converted from one game to another with some work. Also, when downloading mods, be sure that, in addition to the right game, they were created for the right patch level (you can find yours out in-game by selecting Advanced and then About This Build). You can always download the right patch from the Patch & Updates category of the database.
Installing Mods
Nearly all mods can be installed relatively easily, without any technical work. First, navigate to your C:Program FilesFiraxis GamesCivilization4, but NOT to the ../My Documents/My Games/ folder. Then, depending on which game you’re playing, open either directly the mods folder, Warlords, or Beyond the Sword folder. In all three cases, open the MODS subfolder. After you download the mod’s ZIP file, extract the mod’s main folder (almost always named the title of the mod) to this MODS directory.
Once in game, select from the Main Menu Advanced and then Load a Mod. Choose the mod from the list of mods available, and just wait for the game to restart, as it needs to load the changed game files. Then you’re all set! If you encounter problems, read this thread to double-check your procedure.
In some cases, mods will require a non-standard installation. In this event, you can find detailed instructions either from a readme file in the mod’s folder or from the mod’s thread on the forums. Even more lengthy and comprehensive instructions can be seen in the Tutorials & Reference forum.
Creating Your Own Mods
WorldBuilder
For those of you who played Civ3, the WorldBuilder is much like the out-of-game editor in that it allows you to create maps, place units and cities, edit the “fog of war”, set diplomacy, and other features that are useful in making scenarios and maps. It can be launched only in-game by pressing Control + W when a saved game has been loaded, though it can then create a totally new map. The WorldBuilder is fairly intuitive and the interface remains essentially the same as in Civ4, allowing for individuals to make quick changes or more complex scenarios. You can read more about the WorldBuilder and download a manual in this thread.
XML
XML, or Extensible Markup Language, refers to the format of the game’s data files, which store information regarding things like units’ attributes (strength, movement, hammer cost, etc.), leaders’ characteristics, civilopedia entries, and more. These files are located in the default directory of C:/Program Files/Firaxis Games/Sid Meier’s Civilization 4/Assets/XML and can be opened in a program like Notepad.  Simple changes, like giving a unit more movement points or making terrain have a different food yield, can be accomplished with XML, as can more difficult things like creating new civilizations. For more about XML and modding in general, consult this guide to Civ4.
Python
Python refers to the more powerful language that is used by the game to control many things, including the interface, advisor screens, and map generation. Python can be used to create totally new features that are not in the default game. The Python API can be seen here, and a set of tutorials for Python can be read here.
SDK
The SDK, or Software Development Kit, allows modders the most direct access to the main Civ4 game files. By downloading and using the SDK, individuals can edit the game’s DLL source code, enabling them to change the game in ways otherwise impossible. Naturally, this is the most challenging form of modding, as it requires a more complex understanding of programming, but you can read an overview and instructions regarding the SDK on the forums.
Modding at CivFanatics
CivFanatics maintains a very vibrant modding community that has even attracted the attention of Firaxis, with some modders having their creations released on expansion packs. Main modding discussion takes place in the Creation & Customization forum, with subforums containing threads for finished files. Numerous mods have their own individual forums in the Project & Mod Development forum as a way to more directly communicate and receive feedback from users. Also, keep an eye out on the homepage here at CivFanatics for updates regarding recent creations!
Additional Information
Credit for much of this overview goes to the many stickies and influential threads in the Creation & Customization forum. That forum serves as a great reference and contains discussion on a much deeper level. If you have any questions, the users there can provide timely answers.