The Fortress-Tower can be visited
Fortress Linz-Warte Edelburga

The Danube Fortress Linz-Warte Edelburga is a historic fortress tower that was part of the Linz Tower Fortification. This complex was built in the early 19th century to protect the city of Linz from enemy attacks. Warte Edelburga is above the Danube slope and was once connected by a wall to the Klause Kunigunde on the riverbank.
History+
The Linz Tower Fortification, also known as the Maximilian Tower Line, originated on the initiative of Archduke Maximilian Joseph of Austria-Este. His experiences from 1809, when Napoleon's advance on Vienna could not be stopped, led to the idea of an imperial fortification system. Although only the fortification of Linz was realized, the concept was based on a ring of individual, strongly fortified towers that covered each other and formed a far advanced line of defense.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://commons.wikimedia.org/wiki/File:Warte_Edelburga.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: 48.3136, 14.2492