You've created a contest to harvest ideas for your product line, the prize is to work for you, and you consider it a selling point that
we don't pay
you?
Give us a reason to want to win. If you're working in proper sprue plastic, giving the winner a bunch of free terrain including their building wouldn't cost much on your end. Or give them the opportunity to pitch their own, non-medieval building idea that they dream of being made a reality.