First of all, there were four strongholds guarding the four directions of the city, which was well protected on all sides by formidable military strength. The next boundary was a water canal all around the city, and in addition the whole city was surrounded by electric wires. The next fortification was of anila, a gaseous substance. After this, there was a network of barbed wire constructed by a demon of the name Mura. The city appeared well protected even in terms of today's scientific advancements.
When Kṛṣṇa arrived, He broke all the strongholds to pieces by the strokes of His club and scattered the military strength here and there by the constant onslaught of His arrows. With His celebrated Sudarśana cakra He counteracted the electrified boundary, annihilated the canals of water and the gaseous boundary, and cut to pieces the electrified network fabricated by the demon Mura. By the vibration of His conchshell, He broke the hearts of the great fighters and also broke the fighting machines that were there. Similarly, He broke the walls around the city with His invincible club.