• Standard - Wood, Lathe & Plaster, Wattle & Daub, Adobe, Sun Dried Bricks. Unless specified otherwise, standard building materials are used for the majority of buildings. They have thatched or shingle roofs, ordinary doors and light shutters at the windows. These building cannot be fortified (Except for Palisades)
• Fortified - Buildings can be fortified by upgrading the walls to field stone, cobbles, kiln fires bricks, or similar materials. Build a Defensive Wall around the perimeter of the building (260gp for forty squares) and add a Fortification Augmentation (300gp). Total of 560gp for an 'average' building. Note this only fortifies the outer shell of the building, if you want individual rooms fortified, you must buy a Fortification Augmentation for each room individually.
• Heavily fortified - a fortified building can be furthered strengthened by adding upgrading the defensive walls to Stone Walls. This costs 260gp per 40 squares perimeter.

Note: If you want a castle, or a really secure home, you will have to add guard posts, a gate house, armoury etc. as well as employ troops to guard the estate for you.