To develop a thought which has already been touched on in these pages, mediaeval fortification was dual in character:  it had either a purely strategical object, in which case the site was chosen with an eye to its military value, whether inhabited or not, or the stronghold or fortification was made to develop an already existing town or site of importance.  Of the second sort was Wallingford, but of the first sort, as we have seen, was Windsor.  Indeed the distinction is normal to all fortification and exists upon the Continent to-day.  For instance, the first-class fortress Paris is an example of the second sort, the first-class fortress Toul of the first.  Again, all German fortresses, without exception, are of the second sort, while all Swiss fortification, what little of it exists, is of the first.

Now where the first category is concerned a waste space is of value, though its dimensions will vary in military importance according to the means of communication of the time.  A stronghold may be said to repose upon that side through which communications are most difficult.

It is true that this space lying to the south of Windsor was of no very great dimensions, but such as it was, uninhabited and therefore unprovided with stores of any kind, it prevented surprise from the south.

The next point of strategic importance on the Thames, and the last, is the Tower.

Though it is below bridges it must fall into the scheme of this book, because its whole military history and connection with the story of England is bound up with the inland and not with the estuarial river.

It was, as has already been pointed out, one long day’s march from Windsor—­a march along the old Roman road from Staines.  This land passage more than halved the distance by river, it cut off not only the numerous large turns which the Thames begins to take between Middlesex and Surrey, but also the general sweep southward of the river, and it avoided, what another road might have necessitated, the further crossing of the stream.

Long as the march is, there was no fortification of importance between one point and the other, and mediaeval history is crammed with instances of armies leaving the Tower to march to Windsor in one day, or leaving Windsor to march to the Tower.

The position of the Tower we saw in an earlier page to be due to the same geographical causes as had built up so many of the urban strongholds of Europe.  It was situated upon the very bank of the river which fed the capital, it was down stream from the town, and it was just outside the walls.  In a word, it was the parallel of the Louvre.

Its remote origins are doubtful; some have imagined that they are Roman, and that if not in the first part of the Roman occupation at least towards the end of those wealthy and populous three centuries, which are the foundation and the making of England, some fortification was built on the brow of the little eminence which here slopes down to the high-water mark.