The Fortress-Tower can be visited
Fortress Linz-Tower XXIV (Maximilianturm)

The Danube Fortress Linz Tower XXIV, also known as Maximilianturm, is a relic of a fortification system built in the early 19th century. It belongs to a former ring of towers that surrounded the city of Linz and served as a defense system.
History+
The idea for this tower fortification, also called the Maximilian Tower Line, originated with Archduke Maximilian Joseph of Austria-Este. His experiences from 1809, when Napoleon's advance on Vienna could not be stopped, inspired him to propose a new imperial fortification system. This envisioned protecting strategically important locations with a line of freestanding towers. Although only the fortification of Linz was realized, the concept was innovative: a ring of fortified and cannon-equipped towers was intended to defend the city by keeping enemy artillery at a distance and providing mutual cover.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://commons.wikimedia.org/wiki/File:Linzer_Turmlinie_Turm_24.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: 48.3228, 14.2973