The dungeon trap would be placed down for defense. It would kinda be like a spring trap but it has trap doors that fall underground. When you go on to clash the next time after a battle you can check your dungeon and see what troops you have collected. ( only some troops can fall barbarians, archers, giants, wizards) When in your dungeon you can access the troops and see if you would like to move them to your army camp. Or you may discard them. You can hold only so many troops in your dungeon but you can upgrade it like the clan castle.
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