A capital is the most important city -- it does NOT have to be where the government sits
I heard this for the first time in my life. From what I know, Capital is the city which is the country ruled from.
Considering that the developers are Turkish I am going give an example from Turkey. The most important city in the country is Istanbul but it is not the capital. Capital is the Ankara, which is the seat of the government. Capital is the Capital, no matter how strong the city is the heart of the country is where the country is ruled from. Which is the capital. Center of the Country.
Considering that the Empire was pretty old and waged war all over the continent, we might say that the Emperor changed his capital due to politics and wars. If you are in war with Vlandians, ist is not illogical to have your capital in the near. They didn't have internet or whatsapp, they had to send messangers to the commanders and it took a while for a messengar to arrive to warzone, which might end up with your army losing the battles/getting ambushed, being to harsh to the conquered people or being to passive. That's why it is important to be not far away from the warzone. I am saying this due to Calradian Emperors being mostly generals.
But i believe the OP is asking for a ''ethnical capital'' or the homeland of the Empire, which is beyond the sea. They came from the Sea thus, Vlandian lands are probably were their homeland and Praven the capital. They brought up Vlandians with themselves, waged war against the Natives of the continent and conquered inwards. In the meanwhile they moved their capital too and moved their armies to the east which means they left the western part of the country mostly unprotected, or only protected by the Vlandian Mercs. Vlandians rose up and took control of the Western part easily.
Empire is like Rome, but also not. The way they rule the armies etc etc are just like Rome. But the government is highly militaristic. They are spending their time mostly on military rather than ruling what they already have. probably all of the senetors were generals or ex generals. The seat of the government is actualy the headquarters of the army.