You made a mistake. The castle needs more bees and it needs feed (nectar or syrup). Shake some more nurse bees in there ..(quickly) and put on some feed. Hopefully the cell will take. That cell needs to be kept at least 90 degrees and the nurse bees need food...........Did I screw up? What do you think?
I don't know what size "castle" you have but a small 1/2 frame size four frame castle needs at least 2-3 soup can size, nurse bees...