The command of a great town, especially a metropolis, is but partially effected by a fortress situated within its limits. In case of a popular revolt, and still more in case the resources of the town are held by an enemy, such a fortress will be penned in and find itself suffering a siege far more rigorous than any that could be laid in an open country-side. On this account the urban fortresses of the Middle Ages are to be found (at least in large cities) lying upon an extreme edge of the walls and reposing, as far as possible, upon uninhabited land or upon water, or both. The two classic examples of this rule are, of course, the Tower and the Louvre, each standing down stream, just outside the wall, and each reposing on the river. But in an active time even this precaution fails, and that for two reasons. First, the growth of the town makes any possible garrison of the fortress too small for the force with which it might have to cope; and, secondly, this same growth physically overlaps the exterior fortress; suburbs grow up beyond the wall, and the castle finds itself at last embedded in the town. Thus within a hundred and fifty years of its completion the Louvre was but a residence, wholly surrounded, save upon the water front, by the packed houses within the new wall of Marcel. A tendency therefore arises, more or less early according to local circumstance, to establish a fortified base within striking distance of the civilian centre which it is proposed to command; and striking distance is a day's march. The strict alliance between Paris and the Crown forbade such an experiment to the Capetian Monarchy, but, even in that case, the truth of the general military proposition involved is proved by the power which Montlhéry possessed until the middle of the twelfth century of doing mischief to Paris. In the case of London, and of a population the wealthier of whom were probably for some years hostile to the Conqueror, the immediate necessity for an exterior base presented itself, and though the distance from London was indeed considerable, Windsor, under the circumstances of that moment, proved the most suitable point at which to establish the fortress. Some centuries earlier or later the exact point for fortification would have lain at _Staines_, and Windsor may be properly regarded as a sort of second best to Staines.