Yep noticed this today, all my custom empire factions had kingdom-looking walls.
What i did to fix it was to add this to my race xmls

  <NewCityHubType>F_Outpost</NewCityHubType>
  <CapitalHubType>F_Outpost</CapitalHubType>
  <DefaultCityWallSet>CityWalls_Fallen_Fence</DefaultCityWallSet>

Might want to add those in the ingame faction creator so it doesnt mess up in the future if patches change stuff.
Also, adding an option in the creator to choose the music theme (as in this line) would be sweet

  <MusicTheme>EasternWardrums</MusicTheme>