To be privately owned
Fortress Linz-Tower XVIII (Katharina)

In Puchenau, in the Urfahr-Umgebung district in Upper Austria, Austria, at an elevation of approximately 472 meters above sea level, is the Danube Fortress Linz-Tower XVIII, also known as Katharina. It is a remnant of the Linz tower fortifications and was one of the normal towers.
History+
The Linz tower fortifications, also known as the Maximilian Tower Line, were built in the early 19th century at the initiative of Archduke Maximilian Joseph of Austria-Este. His experiences from 1809, when Napoleon's advance could not be stopped, led to the idea of an imperial fortification system.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://commons.wikimedia.org/wiki/File:Turm18_-_Katharina.jpg
This description draws on the facts documented in the linked Wikipedia article; Wikipedia content is licensed under CC BY-SA 4.0. Wording and editing: Mobile History.
Location: 48.3208, 14.2546