I am impressed by your historical knowledge, but the story lacks action. In the last chapter, both the Emperor and the Doge get a scene preparing for the negotations, but the actual negotiations are just one paragraph. You can't drive the story just with discussions between the Emperor and his councilors. I want to see what actually happened outside of the councilroom and why it goes wrong (or right). Show, don't tell (and forgive me for using this trite phrase). Also, the dialogues tend to look a bit like this:

I want, says the Emperor.
But, says the councilor.
Yes, but, says the Emperor.
Okay, says the councilor.

I somehow imagine Byzantine councils were far more complex than that . Dialogues are very hard to get right, but that is no reason to keep them so bare.

That are as much feedback as my tired brain is able to manage. I hope they are helpfull. Anway, I am quite curious what this new ray of light will be.