(Photos: Bata Shoe Museum)
Pictured above and below are chopines of Venice--platform shoes worn by upper class women and their imitators during the Sixteenth Century. To learn more about this fashion oddity, Collector's Weekly interviewed Elizabeth Semmelhack, a curator at the Bata Shoe Museum in Toronto.
These extreme platform shoes may have been impractical for everyday life. But to an extent, that was the point. Semmelhack explains that ihey offered upper class women in Spain and Venice a way to show off their wealth. This is especially true in Spain, where women wore their dresses to the tops of their chopines. In Venice, dresses extended to the ground, so the chopine was regarded as an undergarment.
As you can see from these examples, they were very fancy undergarments.
Here's a video of Semmelhack tracing the history and development of what she calls "one of the most extreme forms of footwear ever worn in Western dress."