Castilian Spanish woman,

Castile, especially Old Castile, was an important grain-producing region. The Duero valley, for example, ran through the grain-producing areas of Old Castile.

The connection to military power works roughly like this:

productive agricultural land → food surplus → larger population → ability to feed armies → taxation and state revenue → military expansion

For medieval Castile, however, it would be too simple to say "Castile became powerful because it grew wheat." Its rise also depended on territorial expansion during the Reconquista, control of land and livestock, taxation, cities and trade, political institutions, and the ability to mobilize warriors. In fact, when Castile conquered the Guadalquivir valley in the 13th century, the redistribution of land and movement of agricultural labor had significant effects on grain production and food supply.