Advanced Civ 1.11

Copy the unzipped AdvCiv folder into Beyond the Sword\Mods under Program Files. The path of the mod's Assets folder should then be Mods\AdvCiv\Assets. When the mod is loaded for the first time, it creates a folder My Games\Beyond the Sword\AdvCiv for the BUG settings. To uninstall AdvCiv, you may want to remove this folder (in addition to the one under Program Files).

The above assumes a BtS installation from optical disc on Windows 8 or earlier, though I understand that it works the same way with the GOG.com edition and on Windows 10 and 11.The default Steam version is not fully compatible with AdvCiv, so Steam users may want to revert to the original (Game Spy) version. Steam places the Mods folder in Steamapps\Common\Sid Meier's Civilization IV Beyond the Sword\Beyond the Sword. On Unix-based systems, the mod can only be played through a compatibility layer or virtual machine. (These guides might help, but I haven't tried either of them: MacOS, Linux. These three posts also look pertinent.)

To start the mod through a Windows shortcut, create a shortcut to Civ4BeyondSword.exe, open the shortcut's file properties, and add a space and mod=\AdvCiv to the Target field. In Steam, the mod name can supposedly be set through the launch options.

It is possible to play with more than 18 civs by replacing AdvCiv\Assets\CvGameCoreDLL.dll with this version of the file. However, doing so degrades performance a bit (i.e. makes AI turns take longer) in games with 18 or fewer civs, and the two DLLs are not savegame-compatible with each other.

A pack with more diverse artwork for units and cities can be obtained here: Ethnic Artstyles, put together by @DeepWell. (Does not support the “Animations Frozen” graphical option.)