There are some Celtic / Goidilic units that require you to take over a settlement from a different faction than the Eleutheroi. For example, certain units are available to the Casse if they have level mic 4 or 5 in certain regions. But. They can't build such barracks themselves due to government limitations. However. The Aedui or Arverni can. I think you get the point by now.

Perhaps something similar applies?