To be fair, they aren’t exactly hidden. It’s difficult to hide 50-meter-high concrete behemoths in the middle of a city; they’re taller than all the surrounding buildings with the exception of church spires.
There are six Flaktürme, or flak towers, remaining in Vienna from WWII. They’re huge concrete fortresses built in pairs: each pair contains one anti-aircraft gunning tower and one control tower, connected by underground wires (often in tunnels large enough to double as escape routes) for communication. Five are closed to the public, and most of those are closed entirely — the doorways are bricked over. One is in the closed courtyard of a military base; the Austrian Army still uses it for radar and radio transmissions. That one is the least accessible.
Although they were built primarily as anti-aircraft defense towers, by the time they were completed planes had improved and were able to fly higher than the towers could shoot. As a result, they were primarily used as huge air raid shelters for up to 30,000 people each, as hospitals, and to store precious artifacts and art during the war. The walls and ceilings are between 10-20′ thick, and each tower has its own extensive water tanks or well. They also had a system in place to lower the radar (which was huge — about 22′ in diameter) into the tower to protect it during attacks. Each tower is a different height due to the terrain; their platforms are all the same height above sea level to facilitate radar/firing coverage.
The towers were used in Nazi propaganda throughout the war, as symbols of strength. Had they won the war, they planned to sheath the towers in black marble, inscribed with the names of their war heroes in gold.
When the war ended, it wasn’t possible to demolish the towers without damaging surrounding buildings, and later it may have been possible but was prohibitively expensive. Today the towers are protected as historic landmarks.
The Flaktürme aren’t hidden, but they are carefully downplayed, or simply ignored. Dramatic reminders of not-so-distant history, they’re weathered, graffitied, fenced in, bricked up, left off maps. With the exception of a small, pained attempt to acknowledge the history that’s tucked away in a dark staircase in the zoo, they have no signage. Many postcards include pictures of the areas that contain the towers, but always carefully taken from an angle where the tower doesn’t appear — or it’s digitally removed. After the war, they were painted out of postcards and tourist photos of Vienna, and the tradition stands.
With a little effort, though, you can seek them out and research their history, and have a little self-guided tour. I visited the first one because I had read about it, and I wanted to orient myself with a view of the city from a high point. After experiencing that one, I had to see the others and learn more about these massive elephants in the room. It was a fascinating way to explore Vienna, and see parts of the city that I wouldn’t have discovered otherwise. Here they are:
These towers were the first and last ones that I saw on my tour, two days apart. They were the second pair constructed in Vienna (in 1943-44), and the first ones of the third design variation.
1) L-Tower (Control Tower)
This is the one that was converted to an aquarium. If you want to see it for its history alone, you can pay a small fee and leave a deposit to be trusted with a key to the outdoor grated metal staircase, let yourself in, and walk up to the top.
The view from the top is excellent, because of course the building had to be tall enough (this one is 47 meters high) to defend the city below.
If you look around hard enough (and break the rules just a tiny bit to sneak through part of the zoo to get there), you might stumble on a small display in an interior staircase, or notice a tiny museum in an interior room that appears to be open mainly by appointment. The stairway contains some images of the tower under construction, with careful wording about just who was doing the building — if I remember correctly, the term “prisoners of war” was used, which seems rather a stretch. It emphasizes the role of the tower as a shelter for civilians, and minimizes other aspects, like who exactly built it, to defend against what, and why.
The top of the tower has a huge artwork by Laurence Weiner, SMASHED TO PIECES (IN THE STILL OF THE NIGHT), installed in 1991. It’s a fascinating example of art taking on meaning due to its location. Here’s an interesting interview with the artist.
You can also visit the bunker underneath this tower, which has been converted to a small (and odd) museum about the history of torture. Most of the museum ignores its location, but one room mentions the history of the bunker (again, focusing on its role as a shelter for civilians) and you can see the ventilation system:
2) G-Tower (Gun Tower)
This one is in the middle of an army complex in a dense neighborhood. The complex is gated, so you can’t go inside, and only the top of the tower is visible. It’s used by the army for radio communications and as a data center/storage. It’s also an emergency bunker for heads of state.
These two are the closest together, tucked into a park with a playground and a community garden beside them. They’re also the shortest ones, as the ground is higher where they are. They’re different from the other two pairs; this pair was part of the second design iteration, while the other two pairs represent the third variation. They were the first ones built in Vienna, in 1943.
This one has a community garden nestled beside it, and interesting graffiti on the back. It, along with other towers, was considered for use as a datacenter (to safely store servers), but that idea was abandoned and the tower is either empty, or used as storage by a construction company.
This is the blockiest tower, with the largest footprint. It looms over the playground beside it and although at 42 meters high it isn’t too much taller than the buildings around it, it dwarfs them with its presence. It was briefly used to exhibit contemporary art (oh, to have been in Vienna then!), but was closed to the public “due to a lack of official approval” (I’m fairly sure that means something like “due to making politicians feel uncomfortable”), and is still used for storage by the MAK museum.
These two are the most visible, as they’re in a large open park rather than in a dense neighborhood. They sit at the end of broad tree-lined walking paths, where you might otherwise find a fountain or a statue. They were the last ones built by the Nazis (1944-45), and the most technically sophisticated. They have built-in support beams below their platforms, to make it easier to repair them and to support scaffolding etc.
This one is tall (51 meters) and about 400 meters from the G-Tower. In addition to consideration as a data center, there’s also a proposal to use it as an outdoor movie theater (projecting the film on the tower), but so far nothing has been approved and the tower is empty.
This is the tallest of the Flaktürme, at 55 meters. In 1946 some kids got inside and were playing with matches, and a large quantity of ammunition exploded. The roof of the tower was lifted off and then fell back down on itself, causing significant structural damage. Windows were blown out in a wide radius around the tower as well. I couldn’t find anything about what happened to the kids, but I can only assume it didn’t go well for them.
In 2006 one of the damaged platforms was removed, and the structure was wrapped with steel cables to reinforce it. It’s empty, though one portion is used as a cellphone tower now. It has “NEVER AGAIN” graffiti on one side, which is the most overt acknowledgement of the towers’ history that I could find.