layer one-
Outside: Grass, basic tile (sand, dirt, road, water)
Inside: Walls, flooring, the tileset that is used in setting th eborders of the house (see dwarra's mapping tutorial)
Layer two-
Outside: Trees, buildings, cliffs
Inside: appliances and whatnot (tables, desks, beds, etc.)
Layer three-
Outside- decoration (rocks, shrubs, flowers)
INside- Decoration (pots, wall ornaments, table ornaments, etc.)
Layer Four-
events (people, chests, doors, stairs, etc.)