The key to a battlefield is always available to you *as a hand-based skill* whenever you want to travel back and forth between <img src="/ubbthreads/images/graemlins/beyond.gif" alt="" /> and the battlefield. You can leave goods on the battlefield, return to wherever you were, then go back later and pick 'em up, again. Just remember never to leave items on a battlefield as you change acts in the game, because the battlefield is regenerated at that time. Everything in the old one, attached to the old act you just left, is lost.
