This type consists in three parts: frontal or main part, ceiling and basement. The frontal part is the strongest of the three: it is a vertically set plate having the weapon opening and it is almost completey buried in the concrete. What is left fully visible is infact only a rectangular plate which seems to be the thickest part of the central plate.

I have never been so far given the possibility to completely see the shape of the actual frontal plate, nor to verify the thickness of its remaining biggest part being completely buried in the concrete frontal wall. Anyway, I guess the plate must be much wider and higher than it can be seen. The central rectangular plate is completely visible from the chamber and partially from outside. Although the rectangle is as thick as about 35 cm, the thickness of the invisible part is supposed to be smaller.

The second and the third element of the armor are joined so as to constitue a shell-like shield protecting the gunners and they are visible only from inside. While these two other parts could be assembled once the combat block was finished, the central steel plate having so big a weight and volume, required to be put in its deep position during the construction of the block itself.   

Where the three parts armor was adopted, the thickness of the concrete wall can reach as many as 3 and half metres. More precisely, the most common noted measures are: the concrete thickness of the casemate frontal wall ranges in 210-230 cm. To this thickness further 120-150 cm are to be added due to  the camouflaging concrete layers. Thereafter from the external side of the plate to the outermost opening, the overall thickness is within 330-380 cm. The biggest the casemate or block, the more likely its chambers are protected by three parts armors.

The plan of the firing chamber depends on the type of adopted armor; infact if the armor is a three part one, the chamber plan is rectangular (about 80 cm x 300), but if the armor is a medium thickness the chamber is smaller and trapezoidal (back side: about 150 cm, foremost armored wall: 110 cm, depth: about 115 cm).

Turrets are among the most spectacular and easily recognizable metal structures of the Vallo. They were used everywhere: from western Alps sectors to eastern ones including the defensive works in Slovenia, also around Rijeka (Fiume) and Zadar (Zara) in Croatia. These active observatories are constituted by two steel cast elements: a cylinder and the turret itself which is a massive element whose visible part has the distinctive dome-shaped roof.

The turret inside is reachable from the underground rooms by climbing a shaft dug into the rock by means of a ladder. Vertical shafts serving turrets have variable depth depending on the structure and the depth of the underground logistical part. Normally, the shorter the shaft, the longer the walkways with upward stairs connecting the turret chamber to the underground locals. But it is clear that it is impossible to give a general rule as to the shaft depth and the connecting flight of stairs, since it simply does't exist. Anyway, just to give two examples, it sometimes happens to see a square section shaft of just 2-3 metres at the end of a long flight of stairs with as many as 25-45 steps. In other defensive works a 5-7 metres vertical shaft is reachable at the end of a 4-10 steps stairs. The shaft normally has a square 90x90 cm section but in some cases it may have a broader circular section.

Whatever the section of the shaft connecting the turret chamber with the inner rooms, a steel 1.2 m high cylinder is placed on a concrete surface so as to protect the surface mouth of the shaft. This cylinder is the base on which the turret was inserted. Infact, the upper external side of the base cylinder is suitably rounded so as to fit a proper slot on the turret. Moreover, the turret is fully inserted on the base cylinder and strictly held on it by a few vertical groove-tongue fittings: grooves are well visible on the outside surface of the base cylinder. The cylinder is in turn rigidly held on place by three long iron poles which are deeply driven into the rock. These long screws are placed at the corners of a 1.67 m side equilateral triangle. They have a diameter of 65 mm and the bolts have 120 mm sides. The external diameter of the cylindrical base is 180 cm and its thikness is 12 cm.

The same way like in metal observatories, shooters standed on a grating being simply placed on the square shaft mouth. If the shaft section is different there may be a concrete platform with an opening instead of the steel grating with the trap-door.

The turret is a single steel cast element with four embrasures and it could be armed by one or two machine guns whose shooters were protected by circular shells covering the embrasures as shields. Infact, turrets were not intended for soldiers to use as mere observatories, but their functionality included a certain fire action. Moreover, having to possibly cope with infantery attacks on the surface, turrets had to have a wide 360 degree action range hence they coudn't be entirely buried by concrete or earth. Having to be left mostly uncovered by concrete or rock, turrets must have been equipped by armors whose average thickness is about 20 cm.

What can be seen from the outside is what we normally call cupola and it is just the uppermost part of the turret. Infact most of the turret is actually buried into a broad concrete cone like structure. This surrounding flat concrete cone, which protects and strengthens the metal turret, is as high as the four embrasures level. As already said, since the turret has to be fully inserted on the base cylinder, its buried part actually hosts a huge cylindric hollow which is suitable to precisely hold the base cylinder inside it. Admitting that the turret is inserted into the base almost down to the concrete surface, the inside observatory chamber is consequently smashed and it was as high as only 170 cm. If we consider that the average thickness of the cupola is 20 cm, the outside overall height of the turret must have presumably been about 190 cm.

Being the whole structure round, gunners standing inside had the same protection on 360 degrees. Following the trajectory of an hypothetical bullet, to reach the gunners it should have perforated the usual 3 or more metres of the outside massive concrete cone, the turret and the base cylinder.

As for the thickness of the turret, it was of course bigger where it had to be left nake on the surface, while it was presumably smaller where the turret was buried and protected by the circular concrete structure .

There were indeed a few different models of turrets depending on the metal league (thereafter on weight) and the thickness of their cupolas. Lighter turrets had cupola thickness of 10 cm; heavier had 20 cm thick cupolas. Models could also vary due to their cupolas shape: a model has, for instance, more rounded lines, another has a more squared roof line. Regardless of these little weight or shape differencies, the whole structure concept, the embrasures number and the overall dimensions were the same. Being the slot fitting, for the base cylinder to be inserted in, standard, interchangeability may have been kept and different thickness turrets may have been inserted into the same base cylinder.

Although being mainly used as observatories to guide the artillery shots while observing the attackers' movements, these turrets could be used for relatively short distance fire being able to protect the surface of the bunker; this is why they were almost always built on the highest part of the defensive work at isolated and prominent spots.

In small strongholds of type C defensive systems, it is not rare to observe small stand alone defensive works constituted only by a turret and its logistical underground structure. It is as Italians had ultimately built these small works to enhance the weak observation functionality of the overall stronghold thus complying the directives of the circular number 15000.

The concept is similar to metal casemates: steel pipes were vertically inserted into a shaft dug in the rock and the outermost cylinder was top covered by a steel cupola. The cupola has 6 openings: three big ones on the front side and three backward looking smaller ones.

Besides binoculars, armored foremost openings, in case of fire action, could offer suitable emplacements for a light machine gun, while smaller, horizontal shaped backward looking openings could be for pistols to be used.

The overall structure of the metal box seems simple to describe, but understanding its constituting elements and how they are fixed one into another is quite complicate either to understand while observing as well as to describe. It seems not unlikely that any details i'm about to give may be slightly refined by future observations. Anyway, the following notes about the cast elements of an armored observatory seem good enough to give a sufficient global idea of how they were built.

The most common model of this armored observatory is constituted by some iron cast elements: an underlying steel pipe and a steel cupola protruding from the surface. The pipe is not a constant section single cylinder but is's actually made up by two steel cylinders with different sections (unless the two parts were originally welded to make up a single piece). The deeper element has a circular section with an inside diameter of 90 cm. This element is 2 metres high and its lower back side is round shaped so as to form a narrow door. Infact, like in metal casemates, soldiers had to enter the metal structure through the door at its base and, to reach the observatory, they had to internally climb the pipe by means of a vertical ladder.

In most cases the lower cylinder is connected to the underground structure by a more or less long flight of stairs. But, it is interesting to note that if the defensive work structure didn't allow the adopting of the linking stairs and if the depth of the logistical part required a shaft deeper than 2 metres, a shaft of proper depth could have been dug. The 2 metres cylinder was anyway used on the highest part of the overall shaft and its narrow door was in this case fully useless. The deeper part of the shaft is normally lined up by concrete so as to make up a circular section shaft whose diameter is the same of the metal pipe. The concrete shaft below the metal pipe is usually overcome by an iron clamps ladder.

The previous description serves as one more example confirming that in spite of being steel elements always the same, they could be normally arranged in very many configurations according to the particularities of the terrain.

As said, the upper part of the 2 metres high cylinder is welded to another metal cylinder whose section is not circular as the lower one but it is rounded trapezoidal: its transversal inside axis is 106 cm while its longitudinal (firing direction) axis is the usual 90 cm. This second cylinder is just 45 cm high and its thickness is 10 cm. Lying this upper cylinder on a smaller section cylinder, the step caused by their transversal axes difference is used as a base to emplace the grating on. The grating is where soldiers stood while being inside the observatory chamber whose access was by means of a trap-door.

The cupola is actually made up by three steel cast elements whose reciprocal fittings are described by the following description. The first and most important part constitutes the core of the cupola: it is a steel part whose inside section has the usual rounded trapezoidal shape (inside: 106 x 90 cm) since its tight lying on the 45 cm high cylinder constitutes the observatory chamber and its innermost armor. If observed from the inside, the cupola infact seems to be directly lying on the 45 cm high cylinder and there seems not to be any additional junction lines. This innermost element has a lower part which presumably is as thick as 10 cm, but though such a thickness could host a slot to fit the underlying cylinder, there seems not to be any and both touching faces (core and 45 cm high cylinder) are flat. More precisely the upper surface of the cylinder has indeed two 7 cm thick blades which guided the core fitting upon it. The cupola core and the cylinder are rigidly held in place by four huge nuts working on four steel rods (60 mm diameter) placed along the main axes.

Like for metal casemates, two 45 degrees oblique poles come out from the rear side of the observatory. These two additional  poles are thrusted into the rock. To accomplish its inserting on the poles, the cupola core has two ear-like oblique rings protruding from its external face and cast on its structure itself.  So the core was either inserted on the oblique poles and strictly bolted on the underlying cylinder. Having never found these additional four bolts left after any metal observatory full removal, i deduce that they are not thrusted into the rock and, consequently being not so long as the oblique ones, they work as simple bolts joining the cylinder and the cupola core.

The cupola core makes up the whole inside observatory chamber. Infact the outermost part of the observatory is carried out by a properly shaped cylinder which is inserted into the core by some groove-tongue fittings. This external cylinder constitues the armor of the observatory and it has either the three front bigger embrasures and the other three smaller back openings. Once inserted into the cupola core, the armored cylinder is held in place by some giant saw teeth fittings.

After the end of WWII, partizans and the jugoslav army normally took away any removable steel parts (doors, engines, ventilation devices, circular stairs, armors etc) and normally used the explosive to take away the fixed steel elements. Then, many casemates, turrets and metal observatories were blown up and what now remains is just the shafts into which these structures were set and a certain bunch of spread around concrete blocks. Having most of the main steel parts normally been taken away, how can i undestand from what of a structure remains if it was a metal casemate or a turret or a metal observatory? It sounds an interesting question. 

If one wanted to understand what a hole was dug for, one should of course first look for any useful writings on the inside walls of the underground part. If a writing and an arrow exist, the problem is of course solved. On the contrary, we have to look for other useful elements around the hole. First thing to observe is the position of the shaft with regard to the work structure and the physical disposition of the terrain. Observatories and turrets are placed on the highest spot of the area. Infact, having turrets a 360 degrees action, they had to be isolated and common metal observatories were also placed on the highest and isolated spots. So if we observe an isolated rough hole without any other useful elements, it's much likely to be a metal active observatory or a turret. Once extablished that the hole was not for a metal casemate, if the hole is indeed a vertical square shaft, it surely was the underground access to a turret. Some doubts can rise if the vertical hole is not square and there are no left elements to judge if it was a turret or and observatory.

If the hole is instead placed under the highest parts of the work, if there are other similar structures beside and, above all, if the position with regard to the target requires a maily frontal action, it is much likely to have hosted a metal casemate. Being perfectly concealable, metal casemates were largely used where the terrain is flat. 

However accurate the removal of any steel element should have been, many interesting iron parts may have left around the hole. Infact, fewer problems can there be if the hardest to remove steel elements are left: this is the case of the poles which are deeply thrusted into the rock. As said, most of turrets have a short square shaft but there are cases in which the vertical square section shaft is substitued by a circular shaft. What should be in any case present regardless of the connecting shaft, is the three vertical steel poles placed at the angles of an equilateral triangle. The couple of oblique steel poles laterally on the hole, witnesses to the presence of a metal observatory or a metal casemate. It may happen that the nuts are still on the poles which are in turn still inserted into the ear-like steel rings which have been torn off the cupola by the explosion. If the distance between the two left oblique poles is more than 2 metres; it was a metal casemate. If it is around 1.5 metres or less, the poles were used to fix an observatory cupola. If, besides the oblique ones, there are also one or two vertical steel stumps and their distance is about 1.7-1.8 metres, there might be no doubts to recognize that hole was where the lower cylinder of a metal casemate was placed.