Where It All Goes Down
Oyo, an ancient Yoruba city/kingdom, Nigeria
The play is based on a real incident that took place in Oyo in 1946. Although Soyinka warned critics against emphasizing culture clash in their readings of the play, it's hard not to notice all the tension going on between the British residents of Oyo and the Yoruba population and their traditions.
The British residents clearly don't understand the culture of the indigenous Yoruba—and yet they have a ton of control over day-to-day life (including all that law and order jazz), so they end up barreling over these traditions in their attempts to shape the environment to fit their own vision of what is right or proper.
In a prime example of this kind of barreling, Simon Pilkings tries to stop Elesin's suicide because it doesn't fit with the British notion of morality and propriety. He reacts according to his own values, without really ever stopping to consider those of the Yoruba people.
Cultural insensitivity is also behind Simon and Jane's decision to use some indigenous clothing with heavy-duty spiritual significance as costumes for a masquerade ball. If that doesn't signal that you're using someone else's lives and culture as a playground, what does?
The play toggles between the market, where the traditions of the Yoruba are affirmed and celebrated, and British-controlled locations like the Residency and the Pilkingses' bungalow (where the Yoruba are decidedly not affirmed), so the structure itself plays out this tension between two different worlds and their intersection.
So while Soyinka might claim that the spiritual stuff is really the heart of the action, the culture clash is an important part of the setting/backdrop. There's just no two ways about it.