The Ponttor is one of the two remaining gates of the original city wall of Aachen. It was built in the 14th century and manned by soldiers and militia throughout the Free Imperial City of Aachen era. Today, the Ponttor is used by German Youth and Scout groups.
The Ponttor was constructed as a right-angled three-stock tower castle. In the main entrance, there is a Portcullis as well as a machicolation, through which things could be dropped on invading forces. A bridge passage with crenelations spanned a moat and was strengthened in the foregate with two reinforced towers (barbicans). The building material was Devonian sandstone, Pennsylvanian carbonite sandstone and quartzite, with the framing material made of light bluestone.References:
The city walls of Avila were built in the 11th century to protect the citizens from the Moors. They have been well maintained throughout the centuries and are now a major tourist attraction as well as a UNESCO World Heritage Site. Visitors can walk around about half of the length of the walls.
The layout of the city is an even quadrilateral with a perimeter of 2,516 m. Its walls, which consist in part of stones already used in earlier constructions, have an average thickness of 3 m. Access to the city is afforded by nine gates of different periods; twin 20 m high towers, linked by a semi-circular arch, flank the oldest ones, Puerta de San Vicente and Puerta del Alcázar.