Banner of the Novgorod Republic from circa 1385. 
The Book of Knowledge of All Kingdoms reads the following: 
"This river of Nu bounds a great province called Siçcia, a very cold country. In this Siçcia there is a great city, the capital of the kingdom, called Nogarado. The king has for its device a red flag with a white castle." In the English translation, it refers to Plate 19, 91, which is this flag.

The river of Nu refers to the Northern Dvina River and the city of 'Nogarado' is meant to refer to the city of Novgorod, which itself was the capital of the Novgorod Republic.
Own work. Source: Libro del conocimiento de todos los reinos, (c.1385), n49Other sources: Hubert Herald, English translation of the book, page 58 and page 60
Dekodrak