Alyosha Popovich may have been based on a historical Alexander Popovich of Rostock according to the Nikon Chronicle .
Rostock died in the year 1223 in the Battle of the Kalka River against the Tatars .
Alyosha Popovich may have been based on a historical Alexander Popovich of Rostock , who died in the year 1223 in the Battle of the Kalka River against the Tatars , according to the Nikon Chronicle .