If the Reds did not have Votto, I would think Casas would be a cornerstone of a trade for Castillo. Maybe a third team might get involved.
I doubt we trade Mayer for 2 years of Castillo, but if an extension can be agreed to before the trade is finalizes (does this happen anymore?) maybe we go crazy for him.
It's hard to imaging us finding any package that would land us Castillo without Casas or Mayer.
One possiblity would be if we take Moustakas with Castillo. (BTV has Mike at about -$30M value and Castillo at +68M.)
BTV accepts
Verddugo & Groome
for
Castillo & Moustakas
or
Duran, Downs and Groome for the same two, but it also accepts this unlikely combo:
Duran, Downs, Dalbec & Yorke for Castillo, alone.