Excerpts from Books and Wikipedia
"In April 1520 the communities of Castile rose in rebellion against the heavy taxation being imposed on Spain by Charles V, the Holy Roman Emperor. . . . In 1520 Charles V left Spain to further his ambitions in Europe, leaving the country to be ruled under the regency of Charles's ally, Adrian of Utrecht, the future Pope Adrian VI. . . . The Castilian War of the Communities had started, a period often referred to as the 'Revolt of the Comuneros.' Attempts by Adrian to put down the revolution failed; as the unrest spread and the peasants began to join the rebellion, Adrian's army fell apart. Charles was forced to take action in 1521 by appointing a co-regent who enjoyed the loyalty of the nobility. Despite a defeat by the Comuneros at Torrelobaton, Charles's army was victorious at the Battle of Villalar, and some of the revolutionary leaders were executed. One by one, the towns surrendered to the Imperial Army; the last to hold out was Toledo, where the revolution had started." [Furtado: 1001 Days]