Note: Some of this backstory is speculation of what happened to the previous residents of the region known today as Winden. It may be better explained in the future, perhaps even in game. Although there is no mention of the backstory to Crystal Ruins and Castle found in the game, there is some information that had been shared by Yatterman.
The Story of Winden
There was a time long ago when Winden shared a similar resemblance to the other regions found in PMU. During this era, a vast empire was housed in the region, having full control and ownership over the land. There isn't much information regarding this empire and the kingdom excluding the fact that they were quite hostile people towards outsiders, and did not take kind to the weak or helpless.
Their hostility led to the empire's downfall when the kingdom refused to aid a helpless group of unknown entities. The queen of the empire did see potential power in the entities that could aid her own kingdom in their hard times. She ordered that her guards arrest and interrogate these unknown beings.
These entities had enough of the hostility from the queen and her kingdom. All of the beings cast their own personal curse on the land, people, and the kingdom itself to reveal them for what they truly are; Cold, Heartless, and forsaken... With their curses now engraved on the land and people of Winden, the entities disappeared... As if they were never even there.
Out of fear, the queen made a choice of of desperation to seal her kingdom from the outside world, thinking it would stop the curse. However, as time went on, the climate began to drastically change, and even the people of the empire felt as if their life and soul were slipping away.
This event led to what is seen today as Winden, the remaining remnants of the empire barely even stand to this day, and those who were once inhabitants of the vast kingdom are now only lost souls, cursed to their land for eternity. Some mask the fact that they are no longer even part of the living, still thinking that their kingdom is very much alive. However, there are very few who have embraced the harshness of the endless cold, knowing exactly what they have become, and maniacally loving every moment of it.
The Pokémon that currently resides in the main halls of the ruined castle known as Mamoswine was once the captain of the Castle Guard. Once a loyal bodyguard to the queen, he is now cursed to protect the castle for the rest of his doomed existence. Like many of the Pokémon that were cursed, Mamoswine is still convinced that he is very much alive, and that the empire had never fallen. Yet in reality, he doesn't even truly exist... Mamoswine stood guard in the halls for so long, that he is no longer even living. What holds his spirit together is unknown... It is a possibility that the captain of the guard could have been a different Pokémon before the curse, and that the curse reformed his body into a ruthless, cold-hearted watchdog...
Froslass is the queen of Crystal Castle. Before the curse shattered her mask and showed everyone what she truly was, she ruled over the empire that controlled the region currently known as Winden. She is one of the very few affected who still retains memories of the curse, and even has come to terms with her fate, to the point where she actually enjoys it! She still commands all of the lost souls in the castle. (Excluding one Pokémon...)
Before the curse, she once had Royal Guards who were loyal to her in every way, and they protected her... However, once she reformed, Froslass appointed all of the Ghost Pokémon of Crystal Castle as her true guardians (The Ghosts of Crystal Castle are the only Pokémon that remember and embrace the curse for their own personal gain).