From the article:
Let me now take you on a journey through the design process of the Prithvi.
First of all: this is the brief I have given to Wojciech when it came to sculpting the pieces.
Each type of piece (all Kings, all Queens) must have something that makes them easily recognisable, regardless of which army they belong to (e.g. all queens are clearly female, all Kings have a crown and are sitting on a throne, etc.).