Hammers in three way fight for Spanish defender
The club made a previous bid to sign the 23-year-old full-back last year but his club Celta Vigo turned down the offer.
It is understood the Hammers are keen to reignite their interest in the player before the transfer window closes but would need to get close to the £14m+ valuation being asked by the Spanish first division outfit.
They will also need to see off stiff opposition from Atletico Madrid and A.S. Roma and were the successful Aaron Cresswell would come under intense pressure at the club.
Castro’s agent, Joseba Diaz, says he has had meetings with all three teams and that he and the Spain under-21 international will make a decision about his future in the next week or so.
Having been with his hometown club throughout his career, Castro, who is out of contract in June 2019, is understood to now be keen to progress to a higher level and has ambitions to play in England.
The young left-back, who can also play at right-back, famously scored away at Real Madrid in the quarter-finals of the Copa del Rey last year to knock the Spanish giants out of the competition.