The energy cannon is a massive energy weapon used by Harvester City Destroyers. It is a powerful weapon capable of destroying an entire city and wiping out everything within a 25 kilometers radius. Ironically, the energy cannon is the City Destroyer's main weakness as it is vulnerable without its shields, and any attempt to destroy the cannon while it charges will cause an explosive backlash that completely destroys the Destroyer from the inside out.
Following the War of 1996, the Earth Space Defense salvaged intact energy cannons from the wrecked City Destroyers and reverse-engineered the weapons to develop their own cannons.