I've never heard any conditions on "you can use these regions, but not these".
Unless your Character is called Kerwin, Sordnaz or Kostaja ...then you wouldn't have necessarily heard it. Saying that, I'm really surprised you haven't, as this was made crystal clear throughout the agreement. I didn't want the North eating up the small amount of food Coria manages to produce, not to mention the "accidental" battles which occured at least twice when either BoM or Darka attacked Corian troops within Corian territory when we were supposed to be at peace. That really hacked off at least the Corian military council. It's not hard to order your troops to not be aggressive when in neutral lands.
In regards to the Tara thing, you started marching through without even mentioning it to any Corian. Talk about taking our neutrality for granted. Our allies respected the peace we made with the North, didn't use our lands and didn't attack our nobles. The North was a shambles when it came to upholding their part of the agreement. Hammard troops entered Corian lands on no less than five occasions when they were unwelcome, and Darka and BoM actually
attacked Corian nobles when they were inside Coria!
As we are trying to be an honourable realm, I didn't want to break the peace, so I'm glad the North did, because it just confirmed that you guys couldn't be trusted which is what Saeculo always suspected. (When I refer to the North, I make an exception with Eston). Let's be fair though, we weren't a match made in heaven and Coria has always been rooting for the Empire and her allies.
There have been hiccups within the Central Alliance, but the difference is: Coria, Talerium, Tara and CE actually like one another, we're friends because that's been the traditional history of our realms, or because we were originally CE colonies. If one leader is pessimistic the others 'cheer' them up and try and aid the situation and one another, both militarily and morally.
The Northern and Southern realms have never had the kind of relationship the CA enjoys. The CA is used to fighting Wars with one another, our nobles like one another and importantly our leaders know one another well and have no resentment.
In comparison, the North have a common enemy which has brought them together, which is not the same as fighting for one another as friends. Defenders always have more reason to keep up the fight than those attacking, especially when things start going badly.