Of all that I have read of natural building, your "midden" is in fact key. To this day when you build with straw bale, or cob, some sort of manure is key. Granted it is recommended that you use an animal that only eats plants manure, but...

what do you have to add to the stone walls to get walls good enough for a house?

Seal? Wind protection, weather protection, insect/vermin protection?