Here's a picture of what I'm talking about.
https://i.pinimg.com/originals/65/be/fc/65befcbdb8488dac7d1a732ac5ba6265.jpg
These things were massive, did they have a giant furnace on site that they would use to create these? Were they created somewhere else and shipped? Were they solid iron?
Pardon my ignorance
What you're looking at here is not the entire bunker, but a small part of it, called a cupola. Reverse image search tells me that this is a German one, part of their fortifications in St. Malo. Some information on a similar bunker located nearby This one looks to be a machine gun and observation cupola, as a part of larger concrete bunker or pillbox. The cupola itself extends deep into the concrete, pictured here as a part of a Finnish WWII era machine gun bunker.
Now I can't speak to the origin, design or specifications of this particular piece in your picture. My knowledge is limited to Finnish WWII era fortifications, the Salpa Line, so the following details concern only them. The cupola itself is a separate piece, which has to be cast at dedicated steel works, and then transported to the site. Their immense weight, coupled with maximum capacity of available transportation technology at the time, placed a limit on where these fortifications could be constructed. Smaller and lighter, but also less protective cupolas were designed specifically so that pillboxes could be constructed in unexpected locations, where the infrastructure and/or topography did not allow for larger cupolas to be used.
In the case of the Finnish Salpa Line series of fortifications, the cupolas were made by Oy Wärtsilä AB in their steelworks in Taalintehdas, and by Karhula Oy (now Karhula Foundry Oy) in Kotka. A single observation cupola represented a tenth of the entire construction cost of a single bunker, like the one pictured above. These observation and machine gun cupolas used in the Salpa Line fortifications, could weigh up to 12 tons depending on the specific model and type. The one in your picture appears to be significantly larger in diameter, as it can house two machine guns inside it. These particular Finnish cupolas were cast in a single piece, and were at the time as large as the foundries in Finland could physically manage to produce. These were made of soft-cast steel, which was then annealed to improve its properties in withstanding temperature changes and shell impacts.
These cupolas were indeed solid steel, as both they and the reinforced concrete bunker had to be able to withstand direct and indirect fire from very large caliber guns. The thickness of the cupolas used in the Salpa Line was up to 300 millimetres on all sides. The cupola, as the entire bunker itself, was built to "Strength Class I", which was defined in the Finnish literature with being designed to be able to withstand the following: "Continuous indirect fire from 305 mm artillery, direct fire from 210 mm artillery, singular 420 mm artillery shells, 500 - 1000 kg aerial bombs".
These were installed by creating a wooden support structure on which the cupola was then lifted and carefully placed. Normally the support structure was built in conjunction with the steel framework used for the reinforced concrete, and then the concrete was poured in afterwards. When the cupolas could not be delivered in time, a slot was left for them to which they could then later be placed into and the construction finished.
Understandably, transporting and installing these massive slabs of steel was the most precarious stage of construction, due to the difficult terrain where the Salpa Line was being built to. Small mistakes in fastening them during transport, or in the very final stages of moving them into position could cause the entire thing to crash several meters down through the support structures, or worse.
Some contemporary pictures from the national archive:
The machine gun cupola of a pillbox being lowered on to the support structures. 1 2 3
Observation cupola having been lowered into the steel framework, with concrete pouring in process. 1
Sources:
Arimo, R. (1981) Suomen linnoittamisen historia 1918 – 1944. Helsinki, Otava.
Lagerstedt, J. (2012) Salpalinja - Sotahistoriallisten kohteiden arkeologinen inventointi. Helsinki, The National Board of Antiquities.
Törylä, J. (1995) Bunkkeri. Vantaa, Rakennusalan Kustantajat.
The use of heavy, metal fortifications such as this were not that uncommon in the period, and eye-balling the photo of the emplacement that you found, that doesn't even look that big. They were of course not common off the cuff, but metal emplacements were a feature you could find in fixed defensive projects such as the Maginot Line or the Siegfried Line, both of which used well in iron and steel extensively in their construction, but even by then were nothing new, with cast iron emplacements in fortifications quite common by the mid-19th century, and were being replaced with steel in the late 19th (your picture is almost certainly a steel turret, not an iron one).
On the Maginot Line the best recognized example of these are probably the cloches, which were steel constructed turrets atop the large, cement-construction casements (and hence being very visible). The cloches had several different types. Some were observation turrets where spotters could direct fire for artillery hidden better out of sight, while others housed machine-guns, mortars, or anti-tank guns. Many were constructed to be retractable as well (diagram of the mechanism). Depending on the type they were 1.5 to 3 meters wide, and made of ~30cm thick steel, they were cast at a foundry and moved by rail for installation in their final home. The necessary facilities to do on site would have been far more trouble then it was worth, as this method wasn't really all that hard to accomplish. Once on site, the actual placement of the cloches themselves were accomplished with large cranes, either vehicle mounted or placed. All together some 150,000 tonnes of steel were used in the construction of the Maginot Line.
This only speaks to the Maginot Line, but generally it wouldn't be all that different for other fortifications of the period, such as /u/axter's answer below (or above) in the thread.
Sources
Allcorn, William. The Maginot Line 1928-45. Osprey Publishing, 2003.
Kaufmann, J. E.. & Kaufmann, H. W.. Fortress France: The Maginot Line and French Defenses in World War II. Stackpole Books, 2007.
Potocnik, Aleksander et. al. The Maginot Line: History and Guide Pen & Sword Military, 2011.
Spedaliere, Donato. & Donnell, Clayton. Maginot Line Gun Turrets: And French Gun Turret Development 1880–1940. Bloomsbury Publishing, 2017.
These armoured cupolas were factory-made. Krupp was the biggest maker, but other companies made them as well. They're made of steel armour (i.e., a high-strength steel), and the above-ground portion is about 25cm thick. The most common model of this type was the Sechsschartenturm 20P7. "Sechsschartenturm" can be translated as "six embrasure tower" or "six gunport tower". These cupolas are smaller than many people would expect from a photo with nothing to judge the size from. Two cupolas with people and cars for scale:
show that they would be quite cramped if manned by the maximum crew of 5 (consisting of a commander manning the periscope (which emerged from the hole in the middle of the top, and two 2-man machine-gun crews). If two gun crews were present, they could not use adjacent embrasures - at least one embrasure had to be vacant between the two crews to give them enough space.
These cupola usually weighed from about 40 tons to 60 tons, which is heavy, but not too heavy to be moved. The lugs on the side would have been used to lift them with cranes:
The bunkers these were used in were built to standard patterns. The underground structure would be built first, with reinforced concrete, and armoured doors fitted (which could weight 25 tons), and then a cupola like these could be lowered onto it and bolted into place. More reinforced concrete was then added around it, up to ground level. A still photo of this construction escapes me, but this video:
shows a cupola in place with the reinforcing mesh in place around it, ready for the concrete to be poured. A cut-away view shows how these were connected to the main part of the bunker (and also how cramped these were):
Some of these cupolas were moved twice - a first time from the factory where it was made to the Siegfried Line, and then from the Siegfried line to be used in coastal fortifications in Normandy or Norway, or elsewhere. Thus, fortifications in Norway can have cupolas that were made before the war:
Further reading:
J. E. Kaufmann and H. W. Kaufmann, The Forts and Fortifications of Europe, 1815-1945. The Central States: Germany, Austria-Hungary and Czechoslovakia, Pen & Sword, 2014.
Follow up question: How did defenders deal with spalling effect on the inside of the turret? Looking at these they look like they would be very vulnerable to it. Is it not such a big deal with iron? Did they add an anti-spalling layer on the inside?
