Stoke City have given Leicester City permission to speak to Robert Huth about a possible loan deal.
Ivica Olic looks to be on his way back to Hamburg, with Andre Schurrle increasingly likely to join Wolfsburg.
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.