Whilst the enceinte was in course of erection the Burggraf Frederic VI. sold (1428) to the town the ruins of his castle. Steps were immediately taken therefore to fortify the whole of the Castle grounds with ditch and large revetted circular bastions. Paul Stromer was the director of the works. At this time we first find distinct mention of the Vestner Thor, and the Vestnerthorbrücke. The other main gates, the Neue Thor, the Spittler Thor, the Frauen Thor, and the Laufer Thor had begun to be built about 1380. [Illustration: WALLS AND DITCH] The Wührderthürlein and the Hallerthürlein were constructed probably about the same time as the Vestnerthor--_i.e._ circ. 1430. It was against the gates that the main attacks of the enemy were usually delivered, and they were therefore provided with the most elaborate means of defence. Each principal gate in fact was an individual castle, a separate keep: for it was defended by one of those huge round towers which still help to give to Nuremberg its characteristic appearance. The Laufer, Spittel, and Frauen towers, and the tower near the new gate were built in the above order in their present cylindrical shape (1555-1559) by the architect George Unger, on the site of four quadrilateral towers that already existed. The towers are about 60 yards in diameter. They are furnished on the ground story with one or two gun-casemates, which would command the parapet wall if that were taken. Above, beneath the flat roof, is fixed a platform blinded with wood relieved by embrasures capable of receiving a considerable number of cannon. Guns indeed were in position here as recently as 1796, when together with all the contents of the arsenal they were removed by the Austrians.