Cardiff will allow long-serving Kevin McNaughton to leave this summer along with Danny Gabbidon and Nicky Maynard.
Highly-rated Atletico Madrid midfielder Saul Niguez has signed a new five-year contract.
The 29-year-old was named in Birmingham's development squad for their clash with Coventry on Tuesday.
Castelen is available on a free transfer after leaving Hamburg in June following the expiry of his contract.
The Holland international had a trial at Birmingham's Championship rivals Middlesbrough in the summer, but a move failed to materialise.
Casetelen is now hoping to catch the eye of the Birmingham coaching staff as he looks to win a deal at St Andrews.