Isekai Rohan

Thrud Castle

Humanity's last stand.

The castle named after the red dragon Þrúðr stood tall and strong, among the ongoing construction works. Large red flags and banners decorated its mighty stone walls, depicted upon them, the great red dragon. Mixed in with the banners among the walls were also Einhoren's golden insignia. Representing Her Majesty Victoria's interest in defending the kingdom.

The keep was built large enough to house the thousands of humans working, building and living within. From the distance, Asai could spot a few elves assisting and instructing the human workers. It seems the alliance was indeed already in effect.

One particular elf spotted the man, or specifically, his tailor made military uniform. The man dressed in a mixture of half plate and leather armour immediately ran on down to greet the duke. His long blonde hair, tied into a pony tail, swayed as he jogged on down.

"Your Grace! You're finally here! I'm Leslie, sent here upon the orders of Her Majesty the Queen. To assist you, or well, more specifically to act as your vassal's body guard. However, I regret to inform you, I was late by just a hair's width! By the time I arrived, your knights Robin and Chloe had been reported missing. 

Currently, His Highness, prince Victor has yet to announce whether their status is to be desertion or MIA."

Asai eyed the man from head to toe.

"Greetings Leslie, tell me. Can you fight? Or is your armour just for show?"

"Your Grace, I can fight." Leslie unsheathed his long elven blade, emitting a beautiful sound as it cut the air.

"Good. Leslie, this lady behind me is Violet. She's my vassal, so do me a favour and keep an eye on her safety too." Leslie nodded in affirmation. Returning his blade back into its sheath. "Good. Take point, we shall immediately depart."

Leslie was quick to retrieve his horse from the stables. As he lead the duke through the keep and towards the multiple layers of stone walls, Asai stared upon the new scenery. The mighty red dragon that once slumbered here, blocking the entirety of the pass with its body. The man wondered as to her current location and well being.

"Your Grace, Permit me to ask, but do you have no plans on greeting His Highness?"

"Forget about formalities. I'm here to fight the kingdom's enemies, not to enjoy cups of teas and biscuits with the prince."

Leslie nodded in affirmation. Spurring his horse onwards. The man had spent enough time within the keep digging up information, for the guards and security to recognise him at a glance. No longer having to flash his insignia at every checkpoint. The trio quickly travelled through the multiple layers of walls.

Prince Victor technically was the highest commanding officer currently stationed here. However, Rosemi had already confirmed for him that the prince in reality held the same amount of authority, power and peerage as a duke did. Which was especially so, after Victor Del Lagos' fall from grace.

But as much as a prince and a duke were on the same level. Asai Trichia was still a greenhorn duke, his territory when compared to Duke Jeffrey De Lumix, was only a third of the size. Thus, the power-dynamics between Asai Trichia and Victor Del Lagos was completely undefined.

Leaving it to the whims of the two men, whether or not they would show due respect or attempt to downplay one and another.

Asai, having a penchant for evading political ploys, as he saw them as an inefficient use of his time and energy, decided to just spear-head through the entire keep. If he bumped into the prince, so be it.

In addition to Asai's lack of interests towards politics. He hadn't forgotten the costs of the prince's ambitions for vain personal glory. Approximately 30,000 humans died fertilizing elven soil, because of his desires to prove himself strong. And as Asai had did his utmost to avoid being placed under Victor's command, he naturally felt inclined to once again avoid the man.

...

When Prince Victor was finally informed of the duke's arrival, and quick passing through the territory in which he was the head. The prince felt a glimmer of his past rage surface. He had been slighted greatly, unafforded of the respect a prince of his standing deserved.

Within his eyes, Asai Trichia hadn't just disrespected him, but also the royal family. Thus, the man quickly ordered Istvan and his agents to investigate the young duke.

One particular report that immediately caught his interest.

The duke was seen travelling with an elf. One he personally claimed as his vassal.

"The duke, has multiple elves as his vassals? Chloe, and now this new arrival, Violet? What if there's more hidden away? How is this possible? The man literally returned undefeated within their lands, slaughtering their kind..."

Something began to click within his mind.

"What if, just what if the duke is actually an elven fucking spy!?"

kophzi

Prince, Princess

In France, a duke, or "duc" outranks a prince. In Russia and Austria, the title archduke is an indication of royal blood, and is used instead of prince. 

So, basically it's dependent on the kingdom.

Source: https://www.infoplease.com/world/social-statistics/whos-who-monarchy