"Castillos en el Aire" may not be the right song.
There is a Mexican song called "Las Gaviotas," possibly by Gonzalo Hermosa (an arranger?), sung on an album by the Bolivian singer, Zulma Yugar, and sheet music arranged by Ernesto Belloc, available in a collection at Tulane, but nothing else shows up on Google with "Las Gaviotas, cancion."
It seems to be a trad. song.