As you know, already the first cities on Earth were surrounded by walls and had towers built into them. Fortresses with high walls and, again, towers were also built by the ancient Egyptians (and not only pyramids and temples!), Which were erected on the border of the "land of Nub". Well, the Assyrians became famous for having learned to take such fortresses: special battering rams with archers in turrets destroyed the masonry of the walls, warriors dressed in armor dug under the walls and caused them to collapse. Well, the Greeks and Romans invented all kinds of throwing and wall-breaking machines and assault towers on wheels.
In the Middle Ages, civilization had to re-invent the bicycle in many ways, but what was invented was pretty good in its own way. These are the mott and bailey castles - a special type of medieval castles, which were palisade courtyards: one on a hill, the other, usually next to it.
Such castles were very common in France in the 11th - 12th centuries, and after the Normans conquered England in 1066, also on its territory - in Wales, England and Scotland. The word "motte" is French and means "hill", and "bailey" - English - "castle courtyard". The mott itself was an artificial (or natural) hill made of earth, and the height of the embankment could vary from 5 to 10 meters or more. The surface of the "hill" was often covered with clay or even wooden decking to make it more difficult to climb. The diameter of the hill was at least twice the height.
At the top of such a hill, a wooden, and later stone, tower was built, which served as a home for the owner of the castle, and surrounded by a palisade. Around the hill there was also a water or dry moat, from the ground of which a mound was formed. One could get to the tower via a wooden drawbridge and a staircase on the hillside.
Bailey was a large courtyard with an area of no more than 2 hectares, usually adjacent to the motte, where there were various residential and economic buildings - the dwellings of warriors, stables, a smithy, warehouses, a kitchen, etc. Outside, the courtyard was also protected by a wooden palisade and a moat, but the palisade itself could stand on an earthen rampart.
Mott, with the then military equipment, was difficult to take by storm. There was simply nowhere to put the ram. There were no throwing machines yet, and only suicides could climb the steep slope to the assault. Even if the bailey was taken, it was possible to sit out in the castle at the top of the hill. There was only one problem - the fire hazard of such a castle in extreme heat, when the palisade tree dried up and there were problems with water from the well to water it regularly!
That is why, pretty soon, the tree in such buildings was replaced with stone. But the artificial embankments were replaced with a solid natural foundation, since the weight of such a stone tower, called donjon, was very, very significant. Now the castle looked like a courtyard with outbuildings, surrounded by a stone wall with several towers in the center of which stood the donjon itself - a huge square stone tower!
By the way, what is the difference between a fort and a castle? There are many definitions of both, but there is none that would indicate an exhaustive difference between them. There is a definition, the essence of which is that forts were usually built using earthen and wooden fortifications, and the castle was a stone structure, although, for example, the first English mott castles were just high hills or embankments with palisades of logs installed on them … The forts of the ancient Romans were wooden, in particular the fortifications on the border and around the city of Alesia, which became classics, as well as the forts of American soldiers on the prairies of North America, while medieval castles were eventually built only of stone. Well, the castles themselves over the centuries became more and more complex, but the modest fort remained mostly a wooden fence on an earthen embankment.
All this changed with the advent of cannons, which could cause serious damage to stone walls and castle gates, and from a decent distance. Old castles became obsolete almost instantly, but something was needed to take their place. And here the forts came out on top. Cannonballs were not afraid of their earthen embankments. Moreover, military engineers soon discovered that by combining earth and stone, they could build forts that could withstand any artillery attack, and moreover dominate the terrain. Even when a new, more destructive artillery appeared, firing elongated shells, forts did not disappear into the past, but turned into even more complex engineering structures, protected from direct fire. Many forts had underground rooms for ammunition and soldiers, artillery casemates and "courtyards", inside which there were whole batteries of heavy mortars, which had been targeted in advance at the terrain surrounding the fort. That is, the fort could suppress the enemy with its fire, but his enemy could not!
The “golden age” of forts in Europe was between 1650 and 1750, with some of the forts of the First World War being built during this period (and later renovated and rebuilt). A key factor in the change was the introduction of an efficient hanging fire. The system: glacis, ditch and rampart, provided protection from fire from heavy siege weapons, field artillery and rifle fire, did not provide protection against bombs flying along a steep trajectory. At first, this was not a problem to worry about because of it, as it was extremely difficult to transport heavy weapons to enemy fortresses by horse traction. For example, heavy mortars for the siege of Vicksburg had to be delivered by river. Heavy mortars were delivered to Sevastopol by sea and … the city fell, despite the fact that the defenders had an advantage in the number of flat-fire guns!
By 1870, structures of stone (or concrete) appeared on forts everywhere. Some forts were equipped with underground chambers and passages through which their defenders, without being exposed to shots, could get to any of their points. However … it should be noted that forts themselves have never been a particularly pleasant place to live, even in times of peace. In addition, unsanitary conditions often reigned in them: for example, many French forts did not have special bathrooms until 1917 and even later. Yes, but how are they … the question of the annoying reader will surely immediately follow and the answer will be this: well, as it was generally accepted at that time in many Western countries. There were appropriate containers, which were taken out by horse transport from the forts and emptied in designated places. Otherwise, there could simply be an open urinal for soldiers and a descent for feces into the river.
The development of more powerful cannons and high-explosive shells in the last part of the 19th century began to change the forts little by little. Cannons whose barrels protruded beyond the wall parapet or through gun ports or embrasures had little chance of surviving under fire, even if they did not receive a direct hit. Therefore, more and more often, guns began to be installed on descending carriages. By raising the large counterweight, the gun was lowered and hidden, and when the counterweight was lowered, it rose and fired. But even the descending guns were still vulnerable to overhead fire. Therefore, the idea was born to cover the guns of the forts from above with armored caps. True, there was a problem here as well. There was a danger that relatively minor damage could jam this armor cap, and thus disable a perfectly serviceable weapon.
In some fortresses, the cannons were housed in huge steel towers, similar to the gun turrets of battleships. However, practice has shown that they are all prone to jamming. Fewer guns can be placed in reinforced concrete casemates and fire through the embrasures covered with armor shields. In some cases, the guns could be mounted on rails so that they could be quickly moved to positions, fired and sent back to cover.
The increased power of the shells used by the besiegers were opposed by materials such as steel and concrete. The stone facing of the shafts was replaced with concrete, and all other structures of the forts were also made of concrete at the turn of the 19th - 20th centuries. Machine guns began to be placed in special machine gun mounts built into the main concrete structures of the fort. Sometimes it was just a concrete ring in which two soldiers with a machine gun could be squatting. In other cases, they were prefabricated concrete or metal blocks of bunkers with embrasures in all directions and a hatch in the floor for urgent evacuation.
It is interesting that in Europe the attitude towards forts was different and ambiguous. Thus, Britain tended to rely on its Navy to defend its island from invasion. As a result, with the exception of some coastal fortifications and coastal batteries covering the approaches to naval bases, the British did not have modern forts. Germany, on Moltke's advice, preferred to build railways rather than forts. Therefore, in addition to the Tau Qin Fort in China, all of Germany's forts were intended to protect naval facilities. The United States erected a series of powerful coastal forts, armed with heavy mortars, the shells of which were capable of hitting the unprotected decks of enemy ships. Forts were also built in a number of places of the Ottoman Empire, including on the approaches to Constantinople and at the entrance to the Dardanelles. Turkish forts usually lagged behind life and did not have any cover from hanging fire.
Nevertheless, the forts turned out to be very effective against the united Anglo-French fleet during the Dardanelles operation and, first of all, because … there were no heavy mortars on board the battleships that fired at these forts! On the other hand, the Turkish fortress Erzurum, which defended the route to Western Armenia, had a garrison of over 15,000 soldiers and over 300 artillery pieces. But, despite this, in February 1916, after six days of intense artillery shelling ("Big Bertha" was not needed!) And infantry attacks, she was taken by Russian troops.
Russian history knows many sieges and stubborn defenses, but at the end of the 19th - beginning of the 20th centuries, of course, it was Sevastopol and Port Arthur. The destruction of the forts protecting Port Arthur with Japanese heavy mortars, one might say, was a kind of hint at the fate of fortresses in Europe after some ten years. But many officers at that time for some reason were inclined to view the Russo-Japanese war as a kind of "oddity", "not our kind of war," as one British officer returning from the theater said. However, the Russian fortresses on the western border played a very important role in the First World War, proving, among other things, that neither heavy weapons, nor even poisonous gas, in themselves, play a decisive role in the assault on the fortress!
As for the Italians and Austrians, they built a number of fortresses on the Trentino plateau. The two lines of forts were about 12 miles apart and were called the "Alpine Barrier". Both the Italian and Austrian forts were very similar in design: concrete foundations on which huge cannons were mounted under cast armored domes. The latter had to withstand a direct hit from such a "big gun" as a Skoda 305-mm howitzer, which was considered a "fortress killer". As it turned out, they could not stand them …
In March 1916, the Austro-Hungarians, in order to punish Italy for abandoning their treaty obligations to the Triple Alliance, launched an offensive in the area. The battle lasted three months, but the maximum penetration of enemy forces into Italian territory was only about 12 miles. Seven Italian forts played an important role in repelling this offensive, and although five of them were destroyed during the hostilities (one 305mm shell went through, for example, a concrete ceiling and exploded inside), the Italians were very grateful to them, because if they were not - then they would suffer complete defeat!
France has been a land of fortresses built there for centuries. The belt of forts along the border between France and Belgium was built by the engineer Vauban. By 1914, modern French forts emerged along the border with Germany and Belgium. The forts on the border with Germany were built to support each other in crossfire. That is, they were built according to the so-called cluster system. Thus, the cluster around Verdun consisted of 20 large and 40 small forts and was supposed to serve as a shield for Paris. It is not surprising that in 1916 these forts were subjected to a massive attack by the German army. At the end of the battle, both sides lost more than 400,000 men, which may have provoked mutinies in the French army in 1917. The Battle of the Somme was largely started only to divert German forces from Verdun. As a result, the battle of Verdun lasted ten months, but … the French still survived! But the French forts on the border with Belgium were abandoned, since all resources were sent to the German border. When the German army moved through Belgium, these forts were unable to offer any meaningful resistance. One fort, for example, had a garrison of only fourteen soldiers!
Belgium reacted to the success of the Prussian invasion of France in 1870 and managed to design and build a number of fortresses. These activities were completed in 1890. The Belgian strategy was not to build on the borders, but instead to create rings of forts around the most strategically important cities, such as Liege, which was “ringed” with twelve new forts, and Namur with nine. Antwerp was already fortified: its forts were built to counter the French threat in 1859. They not only defended their cities, but also blocked the routes of the invading army, which could not go further, leaving them in the rear, as they threatened its communications. Given that Belgium had a defensive treaty with England, it was believed that these forts could delay the advancing German army until the British troops arrived to help it!
A flaw in this approach manifested itself in 1914: it turned out that the forts were not capable of defending for quite some time. This was partly a consequence of underestimating the capabilities of German heavy artillery (and most importantly, the ability to transport and deploy its guns in the shortest possible time!), But the forts themselves had serious shortcomings. Reinforced concrete was not used, and the concrete was poured in layers, instead of immediately pouring the monolith. Therefore, the three-meter thickness of the floors was not enough. A heavy projectile that pierced the concrete floors could blow up the entire fort, as, indeed, happened when only one 420-mm German projectile hit Fort Longines. The heavy guns were housed in retractable turrets, which were susceptible to jamming due to minor damage or even simple mechanical failure. But the biggest drawback was that the forts did not have a well-thought-out system of fire support for each other. Therefore, enemy soldiers could easily pass through the gaps between them.
In 1914, the fortresses of Namur were taken within four days, while in Liege the German army was able to slip past its forts, take the city and wait there for their siege weapons. When they arrived, these forts were taken almost as quickly as at Namur.