Do you know anything about them?
HistoryRuins
What is a ruin? We’re running off of “You know it when you see it” at the moment. Ruins should be non-functioning structures of some age, or their function reduced to tourism and the like.
This is the domed stone mosque at Mwana, Kenya. It's about 9km / 5mi east along the coast from Kipini. Sources (in English, at least) seem very sparse, but Swahili Monumental Architecture and Archaeology North of the Tana River by Dr Thomas H. Wilson and the National Museums of Kenya talks about it a bit and can be found (with a free account) here https://www.academia.edu/27507635/Swahili_Monumental_Architecture_and_Archaeology_North_of_the_Tana_River
@PugJesus slave trading station?
A mosque, part of a larger settlement with houses and tombs. I don't doubt that it could have been involved with the Indian Ocean slave trade but so far as I can tell it wasn't specifically dedicated to that