If I understand you correctly, you want to add a new artstyle, this is done in assets/xml/globaltypes like this;
<ArtStyleTypes>
<ArtStyleType>ARTSTYLE_EUROPEAN</ArtStyleType>
<ArtStyleType>ARTSTYLE_ASIAN</ArtStyleType>
<ArtStyleType>ARTSTYLE_SOUTH_AMERICA</ArtStyleType>
<ArtStyleType>ARTSTYLE_MIDDLE_EAST</ArtStyleType>
<ArtStyleType>ARTSTYLE_GRECO_ROMAN</ArtStyleType>
<ArtStyleType>ARTSTYLE_BARBARIAN</ArtStyleType>
<ArtStyleType>ARTSTYLE_AFRICAN</ArtStyleType> </ArtStyleTypes>
Then you have to assign the civs you want to have your new artstyle. Like this in assets/xml/civilizations/civ4civilizationinfos;
<Type>CIVILIZATION_AMERICA</Type>
<Description>TXT_KEY_CIV_AMERICA_DESC</Description>
<ShortDescription>TXT_KEY_CIV_AMERICA_SHORT_DESC</ShortDescription>
<Adjective>TXT_KEY_CIV_AMERICA_ADJECTIVE</Adjective>
<Civilopedia>TXT_KEY_CIV_AMERICA_PEDIA</Civilopedia>
<DefaultPlayerColor>PLAYERCOLOR_BLUE</DefaultPlayerColor>
<ArtDefineTag>ART_DEF_CIVILIZATION_AMERICA</ArtDefineTag>
<ArtStyleType>ARTSTYLE_AFRICAN</ArtStyleType> <UnitArtStyleType>UNIT_ARTSTYLE_EUROPEAN</UnitArtStyleType>
<bPlayable>1</bPlayable>
<bAIPlayable>1</bAIPlayable>
<Cities>
Then you have to open up the assets/xml/buildings/civ4cityLsystem.xml file and add your new cityart;
<ArtRef Name="generic:
my_own_african_city.nif::1x1_01">
<Attribute Class="Era">ERA_ANCIENT,ERA_CLASSICAL</Attribute>
<Attribute Class="ArtStyle">ARTSTYLE_AFRICAN</Attribute> <Attribute Class="Scalar">iBatchGroup:0</Attribute>
</ArtRef>
This has to be done for every new house you want to add.
Remember, all new houses has to be assembled into one .nif file using only one .dds file for the texturing. Both the .nif and the .dds should be placed in assets/art/structures/cities if I remember correctly.
Take a look at this tutorial:
http://forums.civfanatics.com/showthread.php?t=189687&highlight=city+art