I think the fact that Spain has fewer airfields is due to two reasons.
Firstly GA is not really seen as a hobby by many here, more a route to become a commercial airline pilot. So small airfields are not well supported, and tend to be used more by microlights and autogyros than GA planes.
Secondly, Spain has vast areas of land which are unihabited. Most of the larger towns and cities do already have Aena airports. So aside from building maybe a fly park for foreign GA pilots to visit, it's hard to see where any more airstrips would work.
I've never had the chance to visit Castellon. I must do so if it is under threat.