RM Castilla is the most successful reserve side in the history of Spanish football, having amassed more points in the Spanish Second Division than any other reserve side. In 1980, RM Castilla even reached the final of the Copa del Rey, Spain's premier domestic cup competition, where they lost the title to Real Madrid's first team squad.
RM Castilla is considered the top reserve side in Spanish football history based on its achievements in the Second Division and its appearance in the Copa del Rey final, where it lost to Real Madrid's first team squad.