To obtain the Deck the Halls achievement in Orcs Must Die! all you have to do is start the first level. In your spellbook, equip Arrow Wall Trap, the Sword and, The Crossbow. Next place the Arrow Wall Trap and make sure a few orcs are killed by it. If no orcs die by the Arrow Wall Trap, then the achievement won’t unlock. When the hordes of orcs come, equip your crossbow and just hold down the left mouse button. You should be able to make it through all three waves without any difficulty at all.