Dividing the Headless Knights into inconspicuous small groups, they bring the Mechanical Body parts near the Lizardman Territory, allow the Mechanical Bodies to self-assemble, and then destroy the walls... it's that simple.
Frankly, this is a strategy Yila thought of a long time ago... when she was still the Chief Governor of the empire, she had already pondered the question, "If I were the commander of the monsters, how would I lay siege?"
However, until the end, Yila never saw the monsters employ such tactics, and the countermeasures she had prepared never found their use... Instead, she is now the one utilizing this tactic.
By a clever timing, the destruction of the walls and the arrival of the main army occurred almost simultaneously... This is neither a conspiracy nor could it possibly be described as an open plot.
It is merely a utilization akin to brewing tea with gloriously boiling water.
Though simple, it's effective.
"..."
"Lizardmen, I am Satan Ye, your Demon King."
