Besides distance, what factors could contribute to unique societies and cultures?

Question

My guilty pleasure when worldbuilding is creating varied and unique cultures, however, different cultures are often the result of distance between each other (which allows them to develop unique customs and stuff). For the sake of my story I'm hoping these different countries can be closer together, meaning I have to find new barriers between them besides distance. Could mountains work? Maybe a long deep canyon? Perhaps they are all island countries?

For background, the time period is late medieval/early renaissance period, however each culture is kind of at their own pace and some may be more advanced than others. At the time in which my story takes place, all of these cultures have just gotten to formally know each other (maybe within the last 50~100 years?) and have become more connected, which means trading, new ideas, possibly conflict, idk.

Answer 1

Conditions, Conditions, and Conditions

Say one group develops on an archipelago of separate islands, another on inland plains. Different political organization.

Say one group lives in a key trade node, without much agricultural potential. Different economic organization.

Say one group lives in the floodplains of a predictable river, another one follows migratory species. Different economic and political organization.

Say one group lives in mountain valleys, cut off half of the year and fertile the other half. What does that do to them?

On top of that, religion and traditions.

But cultures will die out unless their traditions contribute to survival and prosperity in the environment they live in. In the Middle East, before refrigerators, you need strict dietary laws. In Northern Europe, different ones.

Answer 2

Us and them

The scope of your question is huge and I guess you'll get plenty of great answers.

However, I think you can boil it down to something simple. Us and them.

All you need is the ability to separate some people from others. This can be distance, the food they like, religion or whatever. Each of these don't necessitate a different society or culture. They can facilitate. The change happens either on it's own, just growing steadily with little outside interference. It is working for them so they grow it that way.

The other thing is rejecting the rest. People mingle, but by seeing yourself apart from lots of other groups, us and them, you can keep your identities and grow them into cultures. Even when all else would pressure conformity and one society or culture.

Interestingly people belong to multiple groups and in essence in multiple (micro) cultures and societies. It is easier to differentiate yourself now in this interconnected society as you can find like-minded more easily, strengthening the identities. But in the end, culture and society are arbitrary drawn lines that can easily be drawn differently, identifying more, less or simply different cultures and societies.