The Roman Tower (Römerturm) is one of several towers in Cologne dating back to Roman times. At the time of the Romans the city was called Colonia Claudia Ara Agrippinensium. This was surrounded by a strong wall with a total of 19 towers. Remains of this wall and some towers can still be seen today. [German]
Of five town gates, only one is left. The Marientor is a gate dating back to 1445. It consists of a tower with the for the region typical conical roof and a barbican covered by a late Gothic wall-walk.
This pic show the remains of the oldest city wall (11th century) of Erfurt. The depicted wall, generally known as Innere Johannesmauer, had been demolished in 1875. Red and blue street name signs still give a hint which streets were inside or outside the city walls in earlier times.
The Aquileia Gate (Porta Aquileia) was errected in 1436 as part of the fifth ring of city walls surrounding the city of Udine. Entering the city through this gate lead to Via Aquileia where you can walk along six old palaces dating back to the 17th-19th centuries.
An interesting way to learn about the former town wall of Wiener Neustadt. At the Reyergasse 10 I saw this mural depicting the walls around the city. Some parts are still to see. E.g. the Reckturm Tower dating back to 13th century is placed at the north-western corner of the city.
The Reckturm is a tower dating back to 13th century. After its demolition it was reconstructed in the style of 15th century. The name Reckturm (reck = rack) refers to its use as prison and place for torture. Today it houses a small museum for weapons and about torment.
The town of Friesach is still surrounded by a water-filled moat. The structure on the left side is a reconstruction of a Bäckertauche. In earlier times a Bäckertauche was used as a punishment tool similar to a cucking stool.