In 1236 Genghis Khan's son Batu Khan, with the fierce general Subutai, led the invasion of what was the state of Kievan Rus (the namesake of modern Russia). Like medieval Europe, Rus was decentralized and fragmented in competing principalities like Moscow and Novgorod, ruled by princes who often waged war on one another. This fragmentation, along with the ferocity of Mongol warfare (entire populations were slaughtered or sold into slavery) likely made the Mongol conquest easier. Fearful, Rus' princes acknowledged Mongol supremacy and agreed to pay tribute, and each local ruler was assigned a Mongol overseer to ensure taxes were collected and rebellion was stamped out. Batu Khan established the Mongol capital at the city of Sarai, where Rus princes came to pay tribute and where Batu's successor, Berke, invited Muslim scholars to settle.
You've arrived after the Golden Age of Batu Khan, but even almost 100 years after the founding of Sarai as the capital, Ibn Battuta will still call Sarai "one of the finest of cities, of boundless size...and possessing good bazaars and broad streets." To the Mongols, Rus is a source of revenue, and at least 40 cities are built here in the 14th century. The Mongols have connected the Rus to markets in the regions of the Volga River and the Caspian, the Black, and the Baltic Seas, along with China, the Middle East, and the Mediterranean. Security and free passage for merchants and goods; privileged treatment for elites, clergy, traders, and artisans; carefully planned tax and land regimes; and mostly indirect governance were the stuff of prosperity, for Russian subjects and Mongols alike (although oppression could be part of this equation, too).
Two of the many mosques of Sarai
Water wheels used for irrigation and for powering mills
An astrolabe, used for astronomical calculations and latitude
Your travels through the Golden Horde are made easier by the introduction of the Mongol Yam postal system. While it's designed to carry messages, your paiza allows you to travel freely and safely through Mongol lands.
Like other parts of the Mongol Empire, the Golden Horde (and Sarai) is diverse. There are Mongols, of course, but also Slavs and many Turks, who will gradually "Turkify" the Golden Horde. Many, but not all, of the Mongol khans of the Golden Horde have embraced Islam, and it will later become the official state religion. Other religions (including the Orthodox Church) were tax exempt under the yassa (Mongol legal code), and Orthodox Christianity actually flourished and strengthened in many areas, including Sarai. For many Slavic peoples, the Orthodox Church represented the Rus national identity and served as a way to distinguish themselves from their Mongol rulers.
While this region does not have the same intellectual renown as China and Persia (Marco Polo first described the people of Rus as "a very simple people"), sitting at the crossroads of east-west exchange has certainly introduced new ideas beyond just religion. There are numerous poets in Sarai, and you even come across an astrolabe similar to the ones you saw at the observatory in Tabriz. Marco Polo pockets that find, as it is useful for navigation.
The historian al-ʿUmari of the Mamluk Sultanate in Egypt described the changes in Russia after the conquest:
"The Circassians [from the Caucasus region], the Russians, and the Alans [from the Caucasus region] are under the rule [of the Golden Horde]. They are people of blossoming and populated cities and wooded and fruitful mountains. Their agriculture thrives and livestock prosper, the rivers flow, and they harvest many fruits. But these people can do nothing against the king and are subject to him, although they are ruled by their own kings [Russian princes]. If they flatter him through subservience and gifts and rarities, he lets them [the princes] rule in peace."
Some of the Russian aristocracy actually benefited from Mongol rule. Princes collected taxes as the Mongol khan's agents, so much of the public did not directly deal with their Mongol overlords. However, while the princes enriched themselves, it was often at the expense and exploitation of the peasants.
A reconstruction of part of the city of Sarai
Caffa and the general slave trade market region and proximity to Sarai
The Genoese fortress at Caffa
The Mongol siege at Caffa
Establishing the capital at Sarai has allowed the typically nomadic Mongols to embrace the settled lifestyle in a prosperous metropolis. Walking through the city, you can tell that the Golden Horde is benefiting from the Pax Mongolica that has stabilized and connected Eurasia, reopening the region to prosperous trade. Even Polo comments on how much has changed since he was in the region twenty years prior, when he found that it was "not a trading land" despite its "great deal of previous furs." Now there are markets, churches, baths, and mosques; and vital trade routes connecting Europe to the East run through these lands. Sarai is situated close to both the Volga River and Caspian Sea, and it has easy access to Tabriz in the Ilkhanate and to Egypt via the Crimean Sea. Thus, the Mongols have invested in maintaining and controlling the yam system through the region.
These trade conditions help explain the recent agreement between the Mongols of the Golden Horde and the Genoese (Italian) merchants of the Black Sea port of Caffa, your next destination. They have secured exclusive trade rights, essentially purchasing the city from the Mongols, and it currently serves as one of the biggest slave markets in the region, in particular supplying slaves from the Caucasus and Eastern Europe for the Mamluks in Egypt. Once again al-ʿUmari provides a description:
"The most numerous of them are the Russians, followed by the Turks of the Kipchak steppe [near the Caspian Sea]. They have many tribes, some Muslim and some pagan. The whole population pays taxes to the ruler of the country [the khan]. Sometimes, when they collect the tax in a bad year when plagues afflicted the flocks or snow fell and harsh frosts ruled, the people sell their children in order to pay them....Although the Tatars [Turks] are superior to the armies of the Circassians, Russians, Magyars [Hungarians], and Alans, those people steal their [the Turks'] children and sell them to slave traders."
You become friends with a few Genoese merchants in Caffa and agree to keep up a correspondence after you return to Italy with Marco Polo. After arriving back home in Venice, Polo ends up in a Genoese prison after joining Venice's war against Genoa. However, you hear he might be publishing a book soon...
As for your friend in Caffa, his letters stop for a time until one day he writes with a shocking story. Jani Beg, the khan of the Golden Horde, had laid siege to Caffa in 1347. They hurled their own corpses, infected with some horrible disease, over the walls, infesting the city. Your friend managed to escape through the route connected the Black Sea with the Mediterranean, but soon after landing in Italy, he falls ill with painfully swollen lymph nodes (buboes) and a terrible fever...
possible spread of the Black Death from Caffa