How do you do it? Because when you are suppressing (as the Suzerain) you send troops to a camp at the enemies city. Allies cannot send to that city (says colonize, but then denies them the ability to send anything) so what, do allies reinforce your Suz city embassy? That would make no sense, since the troops would still be back in the Suzerain city instead of at the camp of the enemy city.

Of course the same thing holds true during an uprising. Both cities can reinforce through their embassies, even though obviously either the Suzerain or Colonies armies are going to the other, but at least you don't actually have to send out a camp on that one.

Very confusing, the Evony makers NEED to publish and maintain a manual for this game, since they change things on a whim constantly. The mechanics aren't even there to reinforce, so at least let us know how we are supposed to subvert them and do it.