The presence of a single subterranean passage at the Tower ought not to have aroused so much surprise, for such "_souterrains_" were a not infrequent feature of the mediaeval fortress. They may be found at Arques, Chateau Gaillard, Dover, Winchester, and Windsor (three), while Nottingham has its historic "Mortimer's Hole." Sometimes they led to carefully masked posterns in the ditches, but they were generally carried along and at the base of the interior faces of the curtain walls, with the object of preventing attempts at undermining, at once betrayed to listeners by the dull reverberations of pickaxes in the rocky ground. There were doubtless others at the Tower, now blocked up and forgotten; indeed, Bayley mentions something of the kind as existing between the Devereux and Flint towers.[58] There is an allusion to them in the narrative by Father Gerard, S.J., of his arrest, torture in, and escape from the Tower in 1597;[59] but the history of the many illustrious captives who have suffered within these walls would in itself suffice for a large volume, while so much, and from so many pens, has already been written thereon, that I have contented myself with few allusions thereto, and those necessarily of the briefest. It is much to be regretted that military exigencies have rendered it needful to remove from the walls of the various prison cells many interesting inscriptions with which their inmates strove to beguile the monotony of captivity, and as far as possible to concentrate them in the upper room within the Beauchamp tower, with which many of them have no historic association whatever; but as the public would otherwise have been debarred from any sight of them, this is far from being the unmixed evil it might otherwise appear, while they have been fully illustrated and carefully described by Bayley. About the time of Edward I. a Mint was first established in the western and northern portions of the outer bailey, where it remained until, in 1811, it was removed to the New Mint in East Smithfield, and the name "Mint Street," given to that portion of the bailey, now commemorates this circumstance. When, about 1882, the extension of the "Inner Circle" Railway was in progress, the site of the permanent scaffold on Great Tower Hill, upon which so many sanguinary executions took place, was discovered in Trinity Square, remains of its stout oak posts being found imbedded in the ground. A blank space, with a small tablet in the grass of the Square garden, now marks the spot.