The list of resources
Saltpeter is required for the first available gunpowder units, and therefore it is of critical importance. It rapidly becomes obsolete, however, as subsequent units do not require it.
When found in nature, saltpeter (also known as potassium nitrate) takes the form of a white powdery substance. A major component in black gunpowder beginning around the 12th century, it has also contributed to the fields of medicine, food preservation and farming. The manufacturing of saltpeter is accomplished by combining the compounds sodium nitrate and potassium chloride.