I'm not sure..

Perhaps a valley of some sorts? Or a mountain area? 

I figure that we would have to have some two legs living close by though so that is why I'm leaning more towards a valley of sorts. It could be in between two mountains and there is only one long thunderpath that cuts through it and a very small amount of humans that live around there. 

As for what brought them together, it could have been a harsh winter where everyone was struggling and then we could bring in Starclan who began to send dreams to certain cats (i.e. the leaders and medicine cats) to form the clans in order to survive.