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Fall of Ro-Lente Castle was the besieging on the Royal Palace of the Re-Estize Kingdom by the Sorcerer Kingdom's forces amidst the war between the two nations. It also saw the assassination of King Ramposa III as well as the defection of Princess Renner to the Sorcerer Kingdom.
Following the Sorcerer Kingdom's siege on the Royal Capital of the Re-Estize Kingdom, all that remained unconquered was the Royal Palace, Ro-Lente Castle. Inside were the last remaining members of the royal family, King Ramposa III and Princess Renner, protected by their knights, who were preparing to make their last stand. With the Kingdom's defeat all but guaranteed, Princess Renner seemed to be planning to commit double suicide with her father to evade capture.
In actuality, though, Princess Renner had been a Sorcerer Kingdom spy as early as the Demonic Disturbance and was, in fact, plotting to assassinate her father as a display of fealty towards the Sorcerer King. While her full motives remain unknown, two possibilities can be drawn: 1. It was a test given to her by the Sorcerer Kingdom; 2. She became worried about her position after the plan to conquer the Kingdom from within was changed into an all-out invasion.
In preparation for the assassination, Princess Renner made sure to remove all possible obstacles to her "play." Although it is unknown if she anticipated it, Blue Roses fled once the kingdom's defeat became certain while Brain Unglaus went ahead to make his last stand during the siege. As a result, the only person-of-interest remaining was the Princess' bodyguard, Climb. She thus send him on a mission to hide the crown jewels throughout the capital to prevent the enemy from acquiring them. In reality, though, the bodyguard was actually placing the jewels in pre-determined locations to be retrieved later by the Sorcerer Kingdom.
On the Sorcerer Kingdom's side, there seemed to be little-to-no preparation for the castle siege, with their main invasion commander Cocytus making a straight path towards the palace; instead merely waiting for Princess Renner to give the green light.
The Princess also successfully carried out her assassination plot by stabbing her father in the back with the royal sword, Razor Edge, right in front of Sorcerer King Ainz and his retainers, Albedo, Demiurge, and Cocytus. Thus proving her loyalty and being accepted into Nazarick as an honorary member.
This operation effectively saw the final dissolution of the Re-Estize Kingdom's central government. Though the Kingdom still had territories remaining that were yet to be subjugated, after the battle the Sorcerer Kingdom spread out its forces to take these as well. Sure enough, without the government's aid, these small independent domains would fall with little-to-no resistance.
Such incident was also served as the final screening test for Princes Renner's admission into the hierarchy of the Sorcerer Kingdom, where the Sorcerer King's three closest aides; Albedo, Demiurge, and Cocytus and the Sorcerer King himself personally take an open analysis on Renner and her overall capabilities as a potential asset or potential threat against their Kingdom should she join. With Albedo and Demiurge having successfully obtained a full grasp on the Former Princess's personality, intelligence, motivations, and overall abilities, this successfully convinces them that they could handle her without much difficulty.
This operation even served to act as the building stage for the Third Princess's personal plot to manipulate her faithful and loving dog, Climb. Once he had been revived by Pestonya Shortcake Wanko, she ensure that he not only remain loyal to her but also have him become loyal to the Sorcerer King upon her revealing her defection to the Sorcerer Kingdom and transformation into an Imp.
During the interval between Climb's "mission" to hide the Royal Treasures and his return to the Royal Palace, Renner had already set up the entire stage for his "Duel" against the Sorcerer King by successfully setting it up as if she was compelled by Demiurge's [Command Mantra] to unwillingly kill her own father and set up his "Heroic last stand" against the Sorcerer King to go smoothly.