The game decided which team would join undefeated Hucknall, Worthing, City of Bristol and Cheltenham Spartans in Championship 2 - and Croydon appeared in the box seat when they led 9-6 at the end of the third period.
That sparked a tremendous Castleford rally in which four goals from Luke Franks and another from Matt Lawtey saw their team squeeze home 11-10.
Croydon drop to Championship 3 along with City of Birmingham and Durham Palatinates.
Photo (Trevor Hyde): Nathan Brown prepares to fire home a penalty for City of Bristol during their victory against Birmingham in men's Division 3.