The island is in the middle of nowhere and is completely uninhabited by Twolegs except for an old blind Twoleg who lives alone on the beach with his herd of goats. It has a beach but the rest of the small island is surrounded by water with steep sides. A few riverbeds and a small stream run through the territory, as well as small Twoleg paths for the goats.
The main thing growing on the island is tall grass and a kind of bay tree that has strange crunchy squishy orange-red berries, but there are also scattered dead white trees and tree-size bushes of heather and wild blueberries. There's also a small abandoned village that's pretty destroyed somewhere in the middle, where the cats' herbs that aren't stolen from the old man grow. The most common herb that grows away from the village and the old man's house is sage.  (I did base this slightly off a real place, there's no way I could come up with all this stuff on my own)

FoamClan-
FoamClan is extremely religious, though instead of the stars they believe their ancestors live in the ocean's waves. They are very wary of the old man, but tend to be peaceful, if a little old-fashioned and overly obsessed with the water and their ancestors. They collect things like shiny rocks and shells, which they pile in a huge beautiful heap in a small hollow that the waves splash into and carry it away to their ancestors.

FoamClan lives in holes under huge old conglomerate rocks piled on top of each other very close to the sea. The smaller rocks sticking out are unstable and can easily break off, do the cats have to be careful when traveling. It's not uncommon for kits to slip on the crumbly rocks and fall into the water. Their main prey is crabs and small fishy things in tidepools.

ThistleClan-
ThistleClan cats are usually taught to be tough and very aggressive. They also stay away from the old man, but for a different reason, the same reason they stay away from and are aggressive towards the other Clans. They're very defensive of their small territory and less plentiful prey.

While there is a place in their territory with a camp very similar to the traditional Clans, since ThistleClan believes that they were the strongest, they actually live hidden deep inside the more spiky parts of the island, where there are the most sharp things like blackberry bushes, random thorny vines, and every other plant is poky. As a result, they have harder paw pads than cats from other Clans and are usually covered in scratches. Also, there are roots sticking out everywhere.  Their main prey is snakes, frogs, some birds, and small mammals. They are the only cats willing to hunt the goats, which is partly why they steer clear of the old man, who doesn't like them either. Their territory also contains a few badger dens, who they respect but will not hesitate to kill if they threaten the Clan.

BushClan-
They're pretty optimistic, lazyish cats with the best territory. Every cat has their own bush with moss woven through the branches to sleep inside and their territory is the sunniest. They generally eat lizards, birds, frogs and other small reptiles that wander in to sun themselves on the collection of rocks they have, but also other prey when they can be bothered to go get it. They have one of the best relationships with the old Twoleg, and he feeds them. They don't have a medicine cat, as they go to the old Twoleg for healing.

The last group of cats don't have a name, but they live on the beach near the Twoleg and are the closest thing to kittypets. He feeds them and they are free to go wherever they like. These cats are usually reckless and rampage through the Clans' territories.