When the beastmen army attacked Strasbourg, Charles had muttered why the Sacred level beastmen didn't infiltrate the city first, then wage war?
At least the Nine Great True Gods' miraculous structures would not have been able to exert their powerful defensive roles.
So when he attacked Black Phoenix, he first used a strategem to draw away the last defense force in the city, and then he didn't directly attack but first had his soldiers disguise themselves and mix into Black Phoenix City. It was only then that he, with all his Sacred level subordinates disguised as civilians, sneaked into the city.
While Charles was leaving the mark of the Serpent of Destiny on the miraculous structures of the Nine Great True Gods, his Sacred level subordinates had already arrived near the Black Phoenix Palace.
The Black Phoenix Emperor, Yannis Hadrian, had recently been suffering from sleepless nights and loss of appetite, worrying every day about how to fend off the allied forces of four nations.