Long story short:
In the 1300s, Moscow, a mediocre(The Kremlin was a wooden fort) city state rose to power because they served as lackeys of the Golden Horde.
A clever Muscovite prince hung out in the Horde’s capital in Sarai, actively currying the Khan’s favor and snapping up all the best opportunities as they came. Quite simply, he was the best at sucking up to the boss.
Moscow’s big break was finally getting appointed as the Khan’s personal tax collector and enforcer over all the other Russian states. Everything Ivan “Moneybags” I did from then on was backed by the threat of the Khan’s armies. Game. Set. Match.