Copa del Rey is a yearly knockout Cup competition in Spain. The winner of the Cup competition gets to play in the prestigious Europa League. It was founded in 1903 and currently, over 126 teams from various divisions participate in the tournament. Barcelona defeated Athletic Bilbao in this season's Cup final to lift their 31st Copa del Rey title.

The Copa del Rey is the oldest Spanish competition to date. Often, there’s a talk in the media about the ‘magic of the Copa’ and how the lower division teams upset the top teams in Spain on various occasions. The Copa del Rey allows teams from lower divisions to progress through the stages to play against top teams like Barcelona, Real Madrid, and Atletico Madrid.

In the Spanish footballing structure, the development teams of the clubs are also allowed to play. But they can't play in the same league as their parent club. The development teams aren't allowed to play in the Copa del Rey as well. However, this rule didn't exist in the past. One such unique incident happened in Spanish football history, which many won't be aware of now. 

The development team of the legendary club Real Madrid which is called Real Madrid Castilla nowadays met their big brothers in the final of the Copa del Rey.

Before 1991, Spain used to treat the development teams or B teams of the clubs equally. They weren't allowed to play in the same division as their senior team but they were allowed to play in the cup competitions (Copa del Rey or Copa del Generalismo) back then.

The Spanish federation wasn't worried about the development teams playing in the final stages of the Cup because these teams used to struggle to even go past the first round. Most development teams used to perform miserably against top-tier sides. During the draws, the development teams used to be separated from the first teams, and the only probability of them playing their senior team but as just once (in the final). This was something the federation never imagined happening. However, it did happen once, in the 1979/80 Copa del Rey season.

The late 1970s and early 1980s are known as the golden era of the Castilla. The legendary Castilla team included the likes of Agustin, Ricardo Gallego, and Fransisco Pineda. Apart from Castilla, 17 other youth teams of various clubs entered the Cup competition that season. But only one team etched their name into the history books, Castilla. 

The Castilla team had very talented young players aged between 20 to 23 years. Even their coach was somewhat young, Juanjo García Santos, just 34 years old. 

The confidence inside the camp was high and Gallego later spoke about the sentiment of the team, "We were a team full of youngsters that went out to play football and this surprised a lot of teams. When a team passes the ball with the fluency that we did when a team runs as we did and when a team has the youthfulness that we did, any opponent that wasn’t prepared would be left behind.”

Castilla blew away Extremadura in the first round of the competition on a 10-2 aggregate over two legs. It was an incredible start to the campaign. They faced Alcorcon in the second round of the competition. Even Alcorcon couldn't give much of a fight to Castilla and eventually lost 5-1 over two legs. Recently relegated from La Liga, Racing Santander couldn't stop the young Blancos in the third round as they lost 3-1. 

Now the Castilla team faces top-tier side Hercules in the fourth round. Hercules defeated them 4-1 at home comfortably and had a three-goal advantage ahead of the second leg. Paco gave Castilla the lead by 2-0 in just thirty mins. However, just one min before full time, Castilla managed to score the third goal, pushing the game into extra time. Cidon scored a 103rd minute winner to send the young Blancos to the Round of 16 of the tournament, the same stage where Real Madrid's senior team arrived through a bye. 

Castilla were to face 23 times champion, Athletic Bilbao, in the Round of 16. Bilbao have always been a team hard to beat in the cup competitions. Bilbao managed to hold the youngsters to a 0-0 draw in Madrid. The advantage was with Bilbao now. In the 2nd leg at San Mames, Bilbao lost the tie 2-1 to crash out of the competition. The scoreline suggests that it would have been a close game but it wasn't. Bilbao was losing by two goals and could only find a consolation goal in the dying minutes of the game.

Castilla faced Real Sociedad in the quarterfinals. After losing the first leg 2-1, Castilla did the impossible by winning the aggregate score of 3-2. This win sounds more impressive when you hear that Real Madrid's senior team lost 4-0 against the same Sociedad side in La Liga that season.

Castilla now faced Sporting Gijon, who finished third in La Liga that season and boasted Quini, one of the best strikers in Spain's history. After losing 2-0 at Asturias, many thought this would be the end of Castilla's progress in the Cup. But the youngsters pulled out a remontada in Madrid in the second leg as they thrashed Gijon 4-0. Castilla did the impossible and reached the final to face their senior team Real Madrid. In an All-Real Madrid final, the senior team had to play their B team at the Santiago Bernabeu. However, here ended Castilla's journey as they lost 6-1 at the hands of their senior team in the final. 

Gallego later added, “The fact we had reached the final meant that Real Madrid were guaranteed to take us very seriously. The truth is that the 6-1 result is a reflection of the distance between us and them.”

As Real Madrid had already won La Liga that season, Castilla were allowed to play in the European Cup Winners Cup next season, a feat no other development team ever achieved.