[2] During his comings and goings to the United Kingdom, he had the opportunity to learn first-hand about the successful growth of professional football, and thus, through the correspondence he maintained with his brother Manuel, he recommended him the convenience of becoming a club.
[2][6] Castellanos took command of the new sports center, which operated with complete regularity, and through their associates, they rented some of the nearby fields of Lamiako to play without problems when the occasion demanded.
[2] Between 1896 and 1900, their only rivals in the city were the crews of the MacAndrews shipping company, British citizens residing in the area for work reasons, and later Athletic Club, which was unofficially formed by Juan Astorquia in 1898.
[12] At the end of 1902, Bilbao FC began to show signs of financial exhaustion due to the non-payment of the appropriate fees by some of its partners who thus compromised the future of the entity.
[2] Once the negotiations had begun, the sudden death of Castellanos in 1903, at the age of just 22,[2][1] precipitated the outcome in a few days, concluding the final agreement on 24 March when both presidents signed the merger in which Bilbao FC was dissolved and all of its remaining members and associates were officially absorbed by Athletic.