The ruins of Minerva

Minerva, also referred to as the Lost City of the North is an ancient city located in the Northern Continent, dating older than most kingdoms. At the moment, the former city is currently being excavated, and so far has produced one picture:

Spoiler:

Located in the Northern Continent, Minerva was a bit ahead of it's time, having high levels of tech for it's time, steam power was normal for Minerva, according to schematics being found in a secure area. However, it was isolated from the rest of the world due to the lack of air or water travel at the time. They would eventually live underground due to the infestation of Nevermores in a nearby mountain terrorizing the civilians, and constant sandstorms blowing through the city, literally burying the city in time.

A few miraculously banded together and created a means of travel to what is now known as Vytal before the citizens of Minerva went underground didn't speak about what happened either, so almost all information was lost, apart from very little tales of a city that was destroyed, roughly 400 years ago. However, a recent expedition led by a hopeful person who believed the tales, stumbled upon the ruined city, leading to it's discovery.

Based on the excavated ruins of the city, Minerva had modern day buildings similar to what Vale would eventually use a hundred years later(300 years ago). Their buildings seemed tall and imposing but due to the ravages of time, have fallen. Literally.

From what was known by ancient texts being translated piece by piece, Minerva was a democratic city, and even had a few Faunus living in a region of the city, dubbed "Zoo", based on a street sign of the same name.

Despite being ancient ruins, powerful Grimm roam the streets, ranging from packs of Alpha Beowolves, Ursa Majors, Boarbatusks, King Taijitus, and Deathstalkers. There are much more, and so it was deemed too dangerous to explore further without help from Hunters. The Grimm in Minerva are far tougher and more coordinated than their Vytal counterparts, due to the harsher environment.

At the moment, progress has been slow in Minerva, due to the powerful Grimm presence in the city. Even with extensive funding, the expedition proves to be difficult with high-end equipment without the aid of skilled Hunters.
Aerial exploration was deemed near-impossible, due to the Nevermores circling the skies of the city.

The history needs to be a little more clear. How long ago was the city prospering? How was it buried?

How is it known that the city was a democracy if all information was lost? What was found to reveal that?

How did this advanced technology come to exist here, before anywhere else?

Updated a few things, set a date, explained how it was buried.