I was thinking of a republic originating from the city of Braavos.
The history of Sinope would go as follow:
Little is known of the birth of the city of Braavos and the Republic of Sinope. It is said that Braavos was founded by refugees from another civilization. They were led by the Sinopians, who settled in the lagoon in an effort to find a place safely away from what is suspected to be the Calibrians. For centuries it was called the Secret City and remained relatively unknown to the rest of the world. Sinope was composed of city-states, the most important being Braavos, Ragousa and Santorini. The relentless spirit of independence and love of freedom allow the Sinopians to defeat the Calibrians (?) in famous battles. Braavos became known in Althena as the land of freedom and refuge. Populations continued to build on several islands of the lagoon, some of which become real cities. These various centres scattered dwellings eventually united and choose the Dogmo as the political center. Braavos started to become a real major urban centre in Althena. The first Doge is named in 205, by the leaders of Relastova and became a protectorate. The lagoon is gradually developing and experiencing its first major crisis when [insert Tretannian leader here] sent a small army to the conquest of Sinope. Resistance, sea control and the situation of Braavos made Trentannia fail in its conquest. Trentannia negotiated Sinope's and more particularly Braavos' future and it was agreed that the region of Sinope would remain under Relastovan influence. It was after these events that the Dogmo began to develop, welcome to political power and form a city that became Braavos. It was the first time the Doge was elected by the citizens. The political power of Braavos on the other city-states of the region of Sinope became absolute in 612 when Doge Plato signed a treaty that would create the Republic of Sinope under the rule of the Doge of Braavos with the most powerful city-states of Sinope to unite against potential Barbarian invasion from the North or from Matinenda or Calibria.
Relastova did not approve of such independence-oriented move by the Doge and started a war against the Republic of Sinope. Relastovians did not even enter Braavos and were pushed back in the Alcadean Sea. A fierce battle on the island of Valessia happaned, resulting in a Sinopian victory. Sinope became completely independent. Its main asset is its position. The lagoon protects Braavos and most of the other cities against enemies and makes it difficult to access. Its location between the East and West of Althena enables it to serve as a commercial bridge between these two regions, ensuring the fortunes of the region. Whenever possible, Sinope remains neutral in disputes between neighbours, but fights against sea pirates and install trading posts in the Alcadean Sea.
Rest to be completed with neighbours
P.S.: Added Braavos' satellite map