The average thickness of the walls of a house, even where they only separated one small room from another, was three feet. They are rarely less than this, and it is only in one or two cases of what appeared to be very important walls that they exceed it, when they reach the thickness of four feet. This measure of three feet was no doubt a well understood one for the wall of a house, and it was continued in the middle ages, when, in ordinary dwellings, only the division walls between house and house were of solid masonry. Municipal regulations then fixed these partition walls at a minimum of three feet in thickness, the cause of which limitation was probably the fear of fires; and in these mediæval municipal regulations, it was further ordered, that closets or cupboards in the wall should in no case be made more than one foot deep, so that if your own cupboard and your neighbour’s happened to back each other, there would still be a foot of solid masonry between the two houses. And the masonry of the Romans may well be called solid. Its character may be seen perhaps to most advantage in the Old Wall above ground. The process of building seems to have been to raise first, gradually, the facings of neatly-squared stones, supported no doubt between frames of woodwork, the supports of which left holes which are still seen in the face of the wall. The interior was then filled up with rubble mixed with liquid and apparently hot cement, which formed the mass of the wall, and in setting has become in course of time harder than the stones themselves. After a certain number of rows of facing-stones, the Roman builders almost invariably placed a string-course of broad thin bricks, the object of which is not at all evident, for they do not go through the wall so as to form real bonding-courses. The Old Wall still standing in probably nearly its original height, will also give us a notion of the elevation of the principal houses of the Roman towns.