It is said the people of Mbo learned the art of building pyramids from Lost Farbosheia. It’s certainly that true that that ancient, inhuman empire greatly influenced many elements of the culture of Mbo. Whether that influence was directly conveyed by remnants of the Yuan-ti or perhaps the new inheritors of a continent just copied the structures they found, ruined though they were, is lost in the mists of time.
Regardless, great pyramids are a centerpiece of most large cities in the Unknown. The styles differ by region. The pyramids of Unknown are smooth sides, the pyramids of the south are stepped ziggurats, the structures of Unknown are three tiered and staired, regardless each major city has at least one, and cities take great pride in their magnificence.
The pyramids are not just decorative however, they serve as the center place for the religion and the priesthood. Each pyramid is typically dedicated to a particular god, usually the patron god of the city, and serves as the center of religious observance, including human sacrifice.
In addition to the great pyramids, which are massive structures hundreds of feet high, the people of Mbo as build lesser pyramids to serve as tombs for the rich and powerful, lesser temples, and such.