The central part of the Modlin Fortress are Defence Barracks (2300 metres long building), they were build between 1832-1844. There are two twin towers on the barracks - Red Tower GC2Q8V7 on the western edge and White Tower on the eastern edge.
Barracks and White Tower are not available for tourist, you can only get to Red Tower in the season. you can only see it from outside, but this is enough to get geocache.
Geocache is in the outer wall of the barracks.