Final answer:
A fortress that dominated historical trading cities along European Russia's rivers is called a kremlin.
Step-by-step explanation:
The historical trading cities along the rivers of European Russia each had a dominant fortress called a kremlin. A kremlin is a Russian term referring to a major fortified central complex found in historic Russian cities. This fortress often housed the city's leaders and served as a military stronghold, a palace, and a religious center. The most famous kremlin is in Moscow, which includes the Red Square and is known for its historical and political significance. The term perestroika refers to the political and economic reforms in the Soviet Union during the 1980s, not a type of fortress. A novgorod is related to the city-states but is not a fortress itself, and an oblast is an administrative division within Russia.