If you are you would know. Let me explain. A few years ago Steam modified its version of Civ IV to work with its own MP system when the old IV MP servers (forget vendor - edit: GameSpy) were terminated. Steam got a lot of backlash as in doing so they modified assets files which conflict with mods. So they released an "original_release" version of the game.
Follow these steps:
1) Fresh install. Let's do this to make sure everything is cleaned up. Delete all files/folders related to Civ IV under Steam/Common and in My documents. (You may have only BTS installed anyway as the Steam version expansions are standalone)
2) Reinstall BTS
3) Do not install any mods
4) Right click on BTS in Steam>Click Properties>Beta tab> on the drop down choose "original_release_unsupported". (Steam will basically reinstall the game to the old version)
In addition, note a few important things about running BTS:
5) Right click the BTS executable in the main folder>Properties>Compatibility>Check box to "run as administrator"
6) Always run BTS from the executable. Never from within Steam. In windows explorer I pin the BTS folder to quick access so I can go right to the folder.
7) You may be aware of this but note that the actually BTS folder is embedded below a higher level BTS folder. The higher level folder is actually the IV vanilla assets. This allows BTS to be standalone. Point is to make sure you are working with the correct files.