Well I doubt the (covert!) company would have untapped guys after going to the site, entering the site, playing an item, facing 3@10, playing another item, and facing again 3@10. If you can pull that off, you deserve to play 3 items in my book, just completely pillage that Dwarfhold I say!
If you want to stay around for another turn, enter again 4@10 and 3@10 to play an item, then I'd say you also deserved it. However, I can also agree with not duplicated at a site, since it is powerful to play items from sideboard indeed. But I do think the card itself should be duplicatable.
First thought of course was to make it for these items of both alignments. But then I thought it would be too limited. And I liked the idea that you can Lure opponent who wants to play Emerald or Enruned Shield. Also the thought of playing underdeeps items somewhere else, yet at considerable danger/difficulty, is appealing, don't you think? At least it's a bit of a surprise, you don't see the Axe in dragon country normally.
Yet, normally I think it will be used for Arkenstone, Axe or Emerald. Iron Crown/Necklace sees little play. The good thing is you can put them in sideboard and watch what opponent does, but even so you're not going to 'guess' opponent wants to play the Iron Crown/Necklace, so will you reserve these cards?
Another thing to consider, if you have Iron Crown in play, nobody can play Fury...
You think Dwarves would like to have the Iron Crown in their hoard? Well, they take anything with some value

Stone-age did not end because man ran out of rocks.