Cats were brought to Iceland by humans circa 870 AD; before the Viking Age, there were no cats on the island. The first warriors were strays who wandered from their human masters to establish a life in the wilderness.
The five Icelandic clans' ancestor guides are believed to be twoleg (read: Norse) gods choosing the forms of cats. The guides are from either StarClan or the Place of No Stars. Neither place, in clan worldview, is necessarily good or evil; they are simply different families, and one adheres to the Warrior Code more than the other.
RainbowClan territory is uphill, with colorful, volcanic silt. Their camp is located behind a waterfall and their namesake is often visible in its spray.
RainbowClan has excellent sentries, with the best eyesight and hearing among the clans. Their tactics involve using sound and motion in misleading or intimidating ways.
They are sudden and unexpected in battle, only making their presence known when it is too late, and using deafening sound when they strike like the horn of Heimdall.
RiftClan can be found in a long fissure and its surrounding fields, littered with uneven, tight stones that would sprain most cats' paws. Surefooted RiftClan hunters excel at flying over this tough terrain like Odin's ravens.
Their camp is located in the cliffside, which their footing helps them to navigate. They are known for their patience and wisdom, camouflaging themselves against the rocks and standing very still until it's time to strike.
GeyserClan cats reside in a meadow of active hot springs, with a camp centered around a heap of boulders. They use the terrain to their advantage, chasing their prey strategically into hot vents or stalking from within steamclouds for stealth. They have been known to go into battle with their pelts gleaming bright from soaking their fur in these same clouds, making themselves luminous like their ancestor Baldr.
GlacierClan uses loose powder snow to slide down from high elevations and land on their prey. These hunting strategies are similar to skiing thanks to their Place of No Stars ancestor, Skadi. GlacierClan cats are also known for their excellent memory and ability to navigate winding passages in the ice. Their camp is a gorge of exposed rock enclosed by icy walls, and is only reached by tunnels through the glacier.
SheerClan spends most of their days traversing sheet ice, requiring them to have quick reflexes, decisive courage, and determination against the ever-changing floes. They are cunning, charismatic and daring, just like Loki, with the ability to swim short distances in the freezing water if needed. Their camp is located within a cove of accumulated flotsam and driftwood.
The Warrior Cats Adventure Game is a tabletop RPG released alongside the "Cats of the Clans" special edition and can be found online as a PDF
Made myself these visuals while using Warrior Cats to learn German (I was gifted "Riesenstern's Rache" ). It helps with recognizing some fairly common terms.
Colored text is used to distinguish the gender, as German has conjugated articles: blue masculine, red/pink feminine, and grey neuter