Have got the nod from the king to open the armoury but there is a red locked chest I can't open it. Is the idol in it? help.

Yes. You need a key. It's hidden elsewhere in the goblin castle. You should have met a goblin who holds a grudge against the king. Check his room.
In case you haven't encountered it before, the red colour indicates a trap. There should be a disarm trap scroll somewhere, but I don't remember exactly where. The king's room maybe?