The Ancient Suwa Civilization was once a known civilization in ancient Japan that used to inhabit the area around Lake Ryujin of the former province of Suwa in the Rurouni Kenshin series.
Though not very much is known about them, their civilization's remains lie deep within the depths of mountain cavities and caverns; the region which Lake Suwa inhabits is also known to be very mountainous, and therefore, the reason why it is rather sparsely populated, but the civilization appeared to be highly advanced in context to this fact to overcome this difficulty and answer a solution to this problem.
Ruins and remains of the civilization showed a vastly stark contrast which it held to more commonly known Yamato historical cultural and aesthetic aspects throughout the nation. Structures all within had details more reminiscent of European and Mediterranean architecture and aesthetics, including statues, arc supported structures, and pillar adorned formations like that of Greece-Roman fashion, foundation and road fashioned out of polished marble, and to a Celtic Cross statue found in its interior. While most structures were fashioned out of wood, and calcified over the centuries, such aesthetics implied that the peoples of this region had right to be known as their own civilization, with even more aspects to be known lost to the ages.
While this civilization still remained lost to the known public all around the world, there were some clues as to how this civilization came to be. One known observation of a Caduceus looking design, made by Schneider of the Black Knights group upon their discovery of the civilization, stated that ancient Germanic and Nordic seafaring peoples once sailed throughout the world and eventually reached the Far East bearing this crest design. It could be implied, reaching Japan, they would have come to use their abundant knowledge collected in their travels, and having settled along with the native peoples of the region, established the civilization of Suwa. Additionally, the European based Black Knights, and the guardian Sanada Ninjas of the region, are likely related, to each other and to this civilization, with both of them sharing similar crests of their respective associations and to this similar crest found within the ruins of the civilization.
As grand as the civilization was to be, ruins and remains left abandoned of the civilization implied once golden times and a zenith before an unspecified cataclysmic decline. One major legacy left behind by these peoples was a species of plant cultivated by them and said to be the very source of eternal youth and immortality spoken of by historical myth all around the world: the Divine Elixir.