The Mechanics of Fortresses An historical journey in the evolution of military science and technique for the construction of the fortresses of Palmanova, Gorizia and Gradisca d’Isonzo.

Fortress of Gradisca d’Isonzo

Video

Episode 1 (duration 2 min 59 sec) Grand Tour between Fortresses (watch the video) – The Lady arrives at Gradisca d’Isonzo, an ancient fortress of the Serenissima Republic of Venice. Here, she finds a wall built of stone blocks, equipped with seven low, thick, circular towers, surrounded by a moat.

Episode 2 (duration 2 min 17 sec) I am Leonardo da Vinci (watch the video) – Leonardo Da Vinci, who at the time worked for the Doge of Venice as an expert in war instruments, knew secrets that could make the difference in war. Informed of the Turkish devastation in Friuli, he arrived at the Fortress of Gradisca.

Episode 3 (duration 2 min 07 sec) The Fortress did not fall (watch the video) – Passed to the Archdukes, the fortress of Gradisca resisted 25 days of continuous bombing. After the war, numerous modernizations were made, including three pentagonal ravelins made of beaten earth and a dry moat.

Episode 4 (duration 3 min 43 sec) A mobile dam (watch the video) – Leonardo, near the Isonzo river, observes the water and begins to draw. He proposes a mobile dam that, in the event of an invasion, would flood the enemy’s forced passages, defending the Fortress. The project was considered too expensive and was never built.

Studies by Alberto Prelli

In 1420 the Republic of Venice conquered the whole of Friuli. This was the premise for transforming the pre-existing village into the fortress city of Gradisca to guard the Isonzo with military and civil functions. In 1472 the Ottomans, having crossed the Carso, easily overcame the weak Venetian defenses, whose troops retreated to Cervignano, the Turkish raiders reached Udine and Cividale, pillaging and destroying villages and countryside. In 1473 the Venetians began building a trench along the Isonzo, with forts at Mainizza and Gradisca.

But, five years later, the Ottoman commander Iskarder Beg passed behind the fortified line and glimpsed Friuli.

At this point Venice ordered, in 1478, Giovanni Emo to transform Gradisca into a real fortress. He entrusted the project to an engineer who traced a regular circuit that enclosed the city.

Military experts of the late 1400s were in favor of polygonal defenses. But initially few rulers took these suggestions into consideration, continuing to build on traditional models.

Between 1496 and 1498 the entire city walls of Gradisca were completed. The walls were built with stone blocks, equipped with seven low and thick circular towers, which could deflect enemy shots and house artillery (the Bell Tower, or Granda, of S. Giorgio, of Portello, the Spiritata, the Marcella, of Calcina, of Palazzo). The city walls were surrounded by a deep dry moat. On a small hill inside the walls, a rectangular building was built, equipped with four towers at the corners, intended to be the Captain’s residence. The new fortress became famous in Europe and was considered capable of resisting heavy artillery fire and responding to fire. It was called Emopoli, in honor of the Emo who had worked hard to build it.

However, in 1499 the Ottomans crossed the Isonzo again, leaving a contingent around Gradisca and out of range of the fortress’s cannons, while other bands of raiders plundered Friuli, reaching as far as Treviso.

Following this devastation, in March 1500 the Senate decided to send Leonardo da Vinci, then in the service of the Serenissima as an engineer, to Gradisca. His task was to inspect the eastern border and suggest a possible line of defense against the Turks. Leonardo thought of a mobile dam, which, in the event of an invasion, would transform the confluence of the Vipacco into the Isonzo into an insurmountable lake, or perhaps be able to surprise the enemy at the obligatory passages by drowning him. The project, due to the exorbitant costs, never came to fruition. Regarding the square of Gradisca, Leonardo proposed the construction of deep merlons on the towers from which to fire with cannons.

In April 1500, Count Leonardo of Gorizia died and Gradisca became part of the possessions of Emperor Maximilian I of Habsburg. A fact that the Serenissima did not recognize.

Venice and the Empire came into conflict. In 1508, the former prevailed, but the League of Cambrai was formed, which brought together the Habsburgs, France, Spain and the Papacy. In September 1510, the coalition attacked the fortress of Gradisca with ladders and 50 small-caliber cannons, but were repelled. A year later it was hit from all sides with medium-caliber cannons (sagri and falconetti). Artillery pieces were also placed on the hill beyond the Isonzo, which hit the inside of the fortress. The Venetian rector was forced to surrender.

Therefore, the city passed into the hands of the Archducals, who began to restore the damaged walls. Inside, where the Venetian castle existed, they built a very fortified castle with large blocks of squared stone.

In this period of time, two problems arose for the defenders of any fortress. First, how to limit the damage of one or more batteries of a potential besieger. Second, how to provide the fortresses/castles with cannons to keep the enemy’s pieces away.

The opportunity for an innovation in the defenses of Gradisca, even if in Italy and Europe the bastion was widely used, occurred in 1615 when Captain Rizzardo Strassoldo understood that Venice was preparing to go to war against the Archduke.

He judged the area of ​​the gate that led to Aquileia to be weaker, filled it internally with earth and also added a large wall. He improved the defenses between the tower of S. Giorgio and that of the Bell. He then had the “old-fashioned” battlements of the curtain wall demolished, which would have been more of a damage than a protection if hit by artillery. In their place he had a large and solid earth parapet built and a high cavalier, on which he placed two pieces of artillery to hit the enemy in the countryside. Above all he had a triangular ravelin built, made with many barrels filled with earth, and equipped it with two cannons. The ravelin, surrounded by a dry moat, in addition to being a further dif

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The jet machines in poliorcetics

Studies by Vinicio Quassi *

Defending oneself from external aggressions has always been one of the main concerns of human beings and it is from this concern that the ability to build fortified places using what has also been called “the architecture of fear” is born.

Defense therefore arises in response to an aggression and, consequently, the improvement of attack capabilities determines the improvement of defensive systems. Kingdoms and empires defended their borders; a spectacular example of this is the Great Wall of China. Men, on the other hand, protected their homes and cities; perhaps the oldest example of this type of defense is constituted by the towers and walls built as early as 7000 BC around the city of Jericho, in Palestine.[1]

The aggressor, to overcome these barriers, had to develop the best siege techniques.

In this brief review we will not deal with the many and heterogeneous siege systems (ladders, mobile towers, battering rams, mine tunnels … hunger, epidemics, betrayal, etc.) but we will take stock of the machines called “throwing” or “throwing”: these are essentially mechanical artillery that used the kinetic energy generated by the effect of tension (of a bow), torsion (of a skein) and gravity (the fall of a weight).

The design of throwing machines probably took place in Greece between the 6th and 4th centuries BC in the period also known as the “Greek miracle”, where technical progress had a significant boost in all fields.

The Greeks considered the devices designed to hurl projectiles as a tool to increase the strength that man did not possess and possibly preserve it in some way. To achieve this, it was necessary to build a device capable of accumulating the energy needed to activate a powerful engine at the right time.

The Greek experience was inherited by the Romans who gave it a further boost up until the threshold of the fall of the Western Roman Empire.

The technical capacity developed in the classical age was partially lost in the transition between the late empire and the early Middle Ages, only to be recovered for further improvement in the centuries of the full and late Middle Ages.

Tension artillery

This type of machine is also called “flexure” because it is based on the bending and release of a bow. To produce enough kinetic energy to effectively hurl a projectile, whether a stone ball or a large dart, the bow must have considerable resistance to stress.

Among the materials found in nature that come closest to the required characteristics, some types of wood are particularly suited, however the result obtained may be acceptable for a simple hand bow but not for a large bow mounted on a carriage.

To solve this problem, the construction technique of the composite bow of ancient Asian tradition is used (fig.1), where the wooden core is supported by animal tendon glued to the back and by horn inserts glued to the ventral part. The tendon counteracts the effect of traction, while the area reinforced with horn counteracts compression. These materials, appropriately modeled and firmly joined, give life to a type of bow with highly significant technical characteristics.

Using this powerful bending element, the Greeks designed the gastraphetes (fig. 2), which is a forerunner of the crossbow, then arriving at the catapult or tension ballista (ca. 5th – 4th century BC), which is basically a large crossbow mounted on a carriage and loaded with a winch (fig. 3). This machine ties its results to the power of the large bow and will always be conditioned by the difficulty of building it in all the eras in which it will be used.

In the late Middle Ages, the tension throwing weapon had its apogee with the large post crossbow (15th century), when research on forging the steel bow was developed (fig. 4).

Torsion artillery

Probably the mechanical complexity of tension artillery determined, in a fairly short period of time, the design of other machines always based on elastic deformation but no longer of a bow but rather of the torsion of a skein made of animal hair and not only.

The new energy accumulator was therefore constituted by an elastic envelope composed not only of horse hair but also of female hair[2] and, later, animal tendons. With the use of the latter the machines were also called neuroballistic. The ballistae appeared mounted on a carriage with the hold similar to that for tension machines but, instead of the bow, they used two independent and rigid arms propelled by as many skeins placed vertically (fig.5); or they could also be mounted on wheels and, in this case, they took the name of carroballistae.

Probably the real target of these machines was neither the single man nor the mighty defensive walls but rather the infrastructures such as the battlements, the roofs of the fortifications and all the structures that were placed inside the walls. The classical chroniclers speak of the launching of enormous rocks, but targeted archaeological research hypothesizes the use of projectiles that reached a maximum of 80 kilograms [3].

The torsion technique, however, continued its path and the need to launch increasingly heavier projectiles led to the construction of single-coil and single-arm machines. We have no certain information on the existence of this type of device in the Greek world, even if traces of their presence are not totally absent. A reference to throwing machines equipped with a single arm is found in Philo of Alexandria who, around the 200 BC, describes such a type of instrument used in the defensive phase.

Three centuries later, the Trajan-era engineer Apollodorus of Damascus left a description of a single-arm stone-throwing system, and around the 4th century AD, single-arm artillery appears, under the name of onager, in the writings of Ammianus Marcellinus and Flavius ​​Renatus Vegetius. In this device, a self-opening sling was placed at the end of the arm for launching, which was in turn moved by the energy imparted by the torsion of a single large coil inserted transversely to the frame (fig.6).

Following this principle, in the Middle Ages the great stone-thrower was developed, a machine to be considered an evolution of the onager and which can be classified as heavy torsion artillery (fig.7). In the figure in question the torsion stone-thrower is “helped” by a large bow.

During the medieval technological revolution, from the mid-12th century onwards, tension and torsion machines were largely abandoned to make room for a new balance-wheel device that exploited the force of gravity.

Counterweight artillery

The history of these instruments begins in China in the 4th – 3rd century BC under the Zhou dynasty. Many scholars agree that in that period the invention of an effective and powerful throwing machine capable of exploiting the force of gravity as a means of propulsion to launch stone projectiles against men and fortifications was placed.

From the writings of the Chinese political philosopher Mozi (Lu, ca. 470 BC – ca. 391 BC) we deduce that the device consisted of a large wooden arm hinged on a frame; an automatic slingshot was fixed to one end of the arm, while a series of ropes were attached to the other end which, at the moment of launch, were pulled simultaneously and violently downwards by a group of servants. The mangle, this was its name, was well known throughout the Middle East from the 5th – 6th century AD and the Byzantines were certainly the main diffusers of this machine in Western Europe (fig.8).

This vehicle had an immediate success and a strong expansion because it was very effective and at the same time simpler, more powerful and safer in construction and use compared to torsion and tension artillery such as the onager and the ballista.

After the 10th century, siege techniques developed further, also due to the castles resulting from the weakening of the Carolingian empire and, in the 12th century, we find references to a more complex mangle that was operated by a counterweight instead of human traction: the trebuchet.

In European armies, torsion machines were largely replaced, during the 13th century, by counterweight ones. The chronicles are not at all precise about the range of these devastating devices, but we could hazard a guess: a 15-meter-high trebuchet with a 10-ton counterweight should be able to hurl projectiles of 5 quintals at a distance of 200 meters.

In addition to the fixed counterweight trebuchet, one with a mobile counterweight was designed which proved to be even more effective and with this weapon we can say that mechanical artillery had reached its peak (fig.9).

The 13th century is not only important for the affirmation of the trebuchet, but also for the report of the presence of muskets, documented for the first time in 1284 on the occasion of the defense of Forlì by Guido di Montefeltro.[4] The advent of the firearm, which probably had as its transitional weapon the blowpipe, first wind and then fire, was an epochal event even if, during the 14th and 15th centuries, bombards and cannons were used at the same time as mangonels and trebuchets. Inevitably, the excellent performances that powder artillery achieved towards the end of the 15th century led to the total decommissioning of counterweight machines. However, the chronicles have a surprise in store for us: in 1521, Hernán Cortes’ troops were besieging the Aztec city of Tecnochtitlan (now Mexico City), but the artillery found itself short of ammunition. In that situation, Private Sotelo, this was his name, showed up and claimed to be able to build a trebuchet to continue the bombardment of the city. Having received authorization, Sotelo improvised as an engineer and erected the gigantic machine. Loaded with an enormous boulder, the counterweight was released, the slingshot was launched with violence but the automatic opening occurred too soon and the projectile, after a vertical trajectory, fell on the trebuchet itself, destroying it.[5]

It is believed that this was the last appearance of a mechanical artillery on the field.

We know nothing about the fate of Private Sotelo.

Bibliography

• Balestracci, Stato d’assedio. Assedianti e assediati dal Medioevo all’età moderna. Il Mulino, Bologna, 2021.
• Contamine, La guerra nel Medioevo. Il Mulino, Bologna, 1986.
• Hogg, Storia delle fortificazioni. I.G.De Agostini, Novara, 1982.

• R. Luisi, Scudi di pietra. Castelli e l’arte della guerra tra Medioevo e Rinascimento. Odoya, Bologna, 2017.
• D. Nicolle, Medieval Siege Weapons (vol. 1) – Western Europe AD 585-1385. Osprey Publishing, Oxford (UK), 2002.
• D. Nicolle, Medieval Siege Weapons (vol. 2) – Bisantium, the Islamic World and India AD 476-1526. Osprey Publishing, Oxford (UK), 2002.
• R. Payne-Gallwey, The Book of Crossbow. Dover, New York, 1995.
• F. Russo, Tormenta. Venti secoli di artiglieria meccanica. Ufficio Storico S.M.E., Roma, 2002.
• A. A. Settia, Comuni in guerra. Armi ed eserciti nell’Italia delle città. Clueb, Bologna, 1993.
• A. A. Settia, Rapine, assedi, battaglie. La guerra nel Medioevo. Laterza, Roma-Bari, 2002.
• G.Todaro, Macchine d’assedio medievali. Le tecniche, le tattiche e gli strumenti d’assedio. Penne e Papiri, Latina, 2003.

*Accademia Jaufré Rudel di Studi Medievali – ETS, Gradisca d’Isonzo (GO).

[1] Cfr. Jan Hogg: “Storia delle fortificazioni” – ed. De Agostini. Novara 1982, pag. 9

[2] Erone, inventore e matematico Greco, uno dei massimi tecnici del settore, dice: “La propulsione dei bracci delle catapulte è così assicurata mediante capelli femminili. Questi, a causa della loro finezza, della loro lunghezza e dalla quantità di olio con il quale sono stati curati quando si costruiscono le trecce, posseggono un’enorme elasticità nonostante forniscano uno sforzo pari a quello dei tendini. Cit. Y. Garlan: “Recherches de Poliorcetique Greque” . Parigi 1974, pag. 217

[3] Cfr. Flavio Russo: “Tormenta, venti secoli di artiglieria meccanica”. Ufficio storico SME. Roma 2002, pag. 187, nota 22

[4] Cfr. Philippe Contamine: “La guerra nel Medioevo”. Ed. Il Mulino. Bologna, 1986, pag.199.

[5] Cfr. G.Todaro: “Macchine d’assedio medievali”. Ed. Penne e papiri. Latina, 2003, pag. 119.