Even without Eston,while Merlin was away from his city and that is HIS mistake ,Darka could easily come in Barad Falas for as long as their units could manage to stay out.
I understand that not everyone has access to the behind the scenes information, and so I'll give you some background info on the state of the allies at the time.
Yep you're correct, it was our own fault for having Merlin leave Barad Falas. But we were offered peace and requested to meet Kerwin in Anost. You tricked us, bravo, well done, it wouldn't have happened again so easily.
Yeah you had 20,000CS in Barad Falas, yet at the time that Kerwin signed peace, we had ~15k Taran, ~8k Corian and ~8k Talerium marching towards the city, around a day or two away. Both allies had just arrived in Coria having undergone a refit. From memory, I don't recall there being over 31K in the city at the time. So the damage you caused having been in our Capital would have been limited to what it had been even if Kerwin had not signed the peace.
The fact is, you've achieved a lot more out of peace negotiations now that Eston has signed the peace then you would have after weeks of fighting. So really, both the allies of Eston and Coria have Eston to be thankful for.
This War has always been exclaimed to be solely against the CE and those that get in the way of your actions against the CE. Peace has been offered on numerous occasions to Coria if we allow access to the CE, so the Darkans have actually achieved what they've always wanted.
The most interesting point I'm going to make is that the reasons justifying the War against the CE in the South is because of it's interference in their foreign politics. Yet this is
exactly what Darka is doing with Eston. A few days after the peace, Kostaja was demanding that Kerwin break the peace and attack Coria again. Hardly the actions of respectful allies.