 Kaloyan , the third of the Asen monarchs , extended his dominion to Belgrade and Ohrid . He acknowledged the spiritual supremacy of the pope and received a royal crown from a papal legate . The empire reached its zenith under Ivan Asen II ( 1218 – 1241 ) , when commerce and culture flourished . The strong economic and religious influence of Tarnovo made it a " Third Rome " , unlike the already declining Constantinople . 
