I recently learned about Igbo pyramids, the Walls of Benin City, and the Walls of Sungbo in Yorubaland. Can you explain anything more about them? Or are there any other great works of African architecture?
EDIT: I just remembered about Great Zimbabwe, Djenne, and Ethiopian monoliths. I didn't think to mention those in my post, since they are so well-known.
One that immediately pops out in my mind is Great Zimbabwe: http://whc.unesco.org/en/list/364
As to why you've never heard about them, I can't really answer that definitively. If you would like to learn more about the empires that existed in Africa and may or may not have created some impressive structures, here is a good springboard.
hi! you may be interested in responses to these other posts
Why were there so few empires in Africa? - as also mentioned by /u/Gewehr98, this post is useful as a reference list
How much do we now know about the Kingdom of Zimbabwe? Is this an active area of study? <-- plug for the BBC doc series Lost Kingdoms of Africa. check it out!
A few questions pertaining to the mysterious Great Zimbabwe. <-- another plug for Lost Kingdoms of Africa
Why have there been so few Sub-Saharan African cities in pre-colonial history?