At the turn of the new century, he seized the strong castle of Constancia (near modern
Simeonovgrad) and then struck in the opposite direction and besieged the last Byzantine stronghold to the north of the
Balkan Mountains, Varna. Varna was defended by a large garrison including western mercenaries who were known to be the bravest soldiers in the
Byzantine army. To take the fortress the Bulgarian engineers constructed an enormous siege tower which was wider that the outer moat. With the help of the siege equipment the
Bulgarian army was able to cross the moat and reach the walls of the city and on the third day of the siege, on 24 March 1201 the Bulgarians made a breakthrough. According to the Byzantine historian,
Niketas Choniates, Kaloyan did not hesitate to kill all defenders despite the fact that it was
Easter. The Byzantines were thrown in the moat and buried alive. According to
George Acropolites, this action was
revenge for the 14,000 Bulgarian prisoners of war blinded by Emperor
Basil II "the Bulgar Slayer" after the
Battle of Klyuch in 1014, when the Bulgarian tsar
Samuil was defeated. The killings earned Kaloyan the moniker "the Roman Slayer". After that he destroyed the city walls and returned to the capital
Tarnovo. == Aftermath ==