RPG Insanity

Hi guys

I completed the game a while ago, but I still cannot figure out how to open the cell door with a teleporter inside it, in the basement to the top right in Next Castle.

Any help would be appreciated.

That teleporter takes you to the Mysterious Cave.  I'm not sure just what opens the cell, but the only item in my inventory that looks related is the Key of Next (found in the Mirror Castle).

Thank you very much, I just found it!