This contribution shows that women played influential roles in medieval warfare not as soldiers on the battlefield but as key figures in communication networks. Two crucial aspects in times of military crisis are gathering information about the enemy and communication between allies. Existing historiography of medieval espionage usually attributes such tasks to men. This article demonstrates, however, that cities systematically used the services of women carrying letters around the front lines and gathering intelligence services about the enemy army. The case study of this article is the war the Flemish cities waged against Emperor Frederick III and his son Maximilian of Austria in 1488–1489, a conflict in which dozens of women as spies and messengers tried to influence the course of the war.