The teams reached the end of the third period at 8-8 but then three goals from Luke Franks and two more from Luke Young took Castleford clear. They will be joined in Championship 2 by Portobello, Worthing, Southampton and Cambridge, with Birmingham - who began the weekend in second place but lost both their matches - descending to Championship 3 with Bridgefield and Ealing.
Bridgefield claimed their first win in their final game - 13-11 against Southampton.