Elven Royal Castle

Elven Royal Castle (エルフ国王城) was the fortress that housed the royal elven family of the Elf Country before falling into the hands of the Slane Theocracy.

Background
The castle was located in the elven capital of Crescent Lake. Nothing much is known about the castle's history, only that it served as the home of the elven royal family. The last member of the elven royal family to dwell in the castle was Decem Hougan. It housed not just royalty, but servants as well as concubines of the Elf King.

The Craftsman of Dwarf Arc
The Elven Royal Castle made a brief introduction as the setting where one of the Elf King's mistresses begged her liege to use his full power against the Slane Theocracy's army that was building a field camp nearby in their war against the elves. She warned him that the camp would lead to the humans attacking the capital in the coming years, but her warnings were ignored by the hedonistic king, as he felt no compulsion to protect weaklings. After he dismissed her from his chambers, the king returned to savoring the sight of his city from his balcony, whilst thinking about his dream of conquering the world with his bloodline.

The Half Elf God-kin Arc
and The Half Elf God-kin II Arc After driving away the members of the Holocaust Scripture, Elf King Decem Hougan returned back to his castle via riding Behemot. Once through the gates he was approached by Myugi, the mother of his slain child, Rugi. When she inquired where their daughter was, the king insensitively stated that she was dead. Oblivious to Myugi's emotional anguish, the male elf ordered her to come to his bedchambers to produce another child.

Decem stopped by the castle's treasury to store his father's equipment and went to his chambers, expecting his mistress to be there, but was surprised when his servant informed him that Myugi had taken her own life by jumping from the castle. The act did not move the king in any way who only saw it as a move by his 'lover' to apologize to him for giving birth to a useless daughter. Decem still needing someone to recreate with, ordered the servant to send his wife and daughters to join him in bed.

During Ainz Ooal Gown first visit to the elven capital, he noticed the castle, in the city, but refrained from investigating it as he did not know how strong the Elf King was and did not want to invite trouble. During the Theocracy's assault on the city the castle was virtually abandoned by staff as they went to assist in laying down fortification against the human invaders. Ainz and his entourage used the cover of battle to infiltrate the castle, in order to perform a kill steal by robbing the castle's treasury. They managed to capture a female elf servant, but after finding that she had no useful information was killed and her corpse dumped through a [Gate] near the Ankyloursus Lord's former territory. The group went through with their infiltration where they found a chamber with floors covered with earth. It was there that Decem confronted the intruders, having sensed their presence.

However he only noticed Aura and Mare, as Ainz was under [Perfect Unknowable]. Upon seeing that they possessed eyes like him, Decem believed that they were of his blood and perhaps grandchildren. He became ecstatic when he felt that they were powerful elves. While Ainz was willing to allow the deluded elf to rant on, his limit was reached when the elf declared his intentions to bare children with Aura and inviting Mare to join in when it. This infuriated Ainz who then punched the perverted elf in the face, initiating a duel between them though Decem was fooled into thinking he was fighting a summon of his 'grandchildren'.

While the two fought, the humans outside the castle were beginning to break through the defenses of the castle. The duel though ended in Decem's defeat having completely played into Ainz's hand. After Decem lost his summon and experience pain for the first time, he lost all his self-confidence and fled in terror back to his room where he hoped to make preparations to flee the castle and build a new kingdom.

But in a strange twist he was met by his daughter, Antilene Heran Fouche, who he begot with Faine who he raped in the past. Having been waiting for him to arrive, though surprised by his terrible appearance, the member of the Black Scripture killed him, finally avenging herself and her mother. Unfortunately her assassination was stumbled on by Mare who was sent ahead to capture Decem. Antilene believing the dark elf was another spawn of the king then targeted Mare. Though she fought formidably, Antilene was defeated in the end and taken prisoner by the Floor Guardian. Aura and Ainz met with him after plundering the treasury and were surprised by the news. Prioritizing to bring their prisoner back to the Frozen Prison for interrogation, Ainz took everyone back to Nazarick leaving the castle to the forces of the Theocracy. However once the prisoner was secured, Ainz summoned Pandora's Actor to retrieve the magic items that were left in Decem's corpse.

Layout
The castle was actually an awfully wide and large elf tree. It was the tallest building within the capital of the elves. It possessed a gate that guarded against intruders.

Features
King's Personal Room: The bedchambers of the king and that included a balcony, providing a view of the city.

Treasury: The castle contains a treasury where the king kept all the nation's and his heirlooms. It was located two floors above the one where the king’s personal room was and it was the highest point in the castle. On the floor where the treasury was, the entire ceiling was about 15 meters high, filled with magical lights covering all of its surfaces. And the entire floor was covered with soil, which acted as a safeguard to protect the floor, as Deceme could use the earth to summon his Primal Earth Elemental.

Trivia

 * Mare's Sunbeams Cloak normally can turn him invisible while in a forest, though in the castle, it was considered indoors even though the castle was made out of trees, reducing the cloak’s ability to the point where even Ainz could sense him more or less.