Castro is one of everal players heading for the exit at Portman Rd.
Manager Jim Magilton is inviting offers for midfielders Gavin Williams and Gary Roberts, defenders Dan Harding and Matt Richards and striker Billy Clarke.
Magilton told BBC Radio Suffolk: "I would not like to stand in the way of them playing first-team football.
"If I can't guarantee first-team football, I think it would be best for both parties if they moved on."
Owen Garvan is said to be mulling over a new contract offer and there's no news on whether Dean Bowditch could also be released.