More laws on mezuzot, specifically placement of the mezuzah on the doorjamb - the Gemara's basic example is of a door on a pivot, not hinges, in contrast to modern day doors. Also, what was one to do if the entrance around the door itself couldn't take nails, etc., to affix the mezuzah? With creative solutions, depending on the material of the doorjamb. Plus, the doorposts need to be there for more than just supporting a portico or rooftop.