Bastion in the context of Hornwork

A citadel is the most fortified area of a town or city. It may be a castle, fortress, or fortified center. The term is a diminutive of city, meaning "little city", because it is a smaller part of the city of which it is the defensive core.

In a fortification with bastions, the citadel is the strongest part of the system, sometimes well inside the outer walls and bastions, but often forming part of the outer wall for the sake of economy. It is positioned to be the last line of defence, should the enemy breach the other components of the fortification system.

Gunpowder is the first explosive to have been created in the world. Popularly listed as one of the "Four Great Inventions" of China, it was invented during the late Tang dynasty (9th century) while the earliest recorded chemical formula for gunpowder dates to the Song dynasty (11th century). Knowledge of gunpowder spread rapidly throughout Eurasia, possibly as a result of the Mongol conquests during the 13th century, with written formulas for it appearing in the Middle East between 1240 and 1280 in a treatise by Hasan al-Rammah, and in Europe by 1267 in the Opus Majus by Roger Bacon. It was employed in warfare to some effect from at least the 10th century in weapons such as fire arrows, bombs, and the fire lance before the appearance of the gun in the 13th century. While the fire lance was eventually supplanted by the gun, other gunpowder weapons such as rockets and fire arrows continued to see use in China, Korea, India, and this eventually led to its use in the Middle East, Europe, and Africa. Bombs too never ceased to develop and continued to progress into the modern day as grenades, mines, and other explosive implements. Gunpowder has also been used for non-military purposes such as fireworks for entertainment, or in explosives for mining and tunneling.

The evolution of guns led to the development of large artillery pieces, popularly known as bombards, during the 15th century, pioneered by states such as the Duchy of Burgundy. Firearms came to dominate early modern warfare in Europe by the 17th century. The gradual improvement of cannons firing heavier rounds for a greater impact against fortifications led to the invention of the star fort and the bastion in the Western world, where traditional city walls and castles were no longer suitable for defense. The use of gunpowder technology also spread throughout the Islamic world and to India, Korea, and Japan. The so-called Gunpowder Empires of the early modern period consisted of the Mughal Empire, Safavid Empire, and Ottoman Empire.

A bastion fort or trace italienne (a phrase derived from non-standard French, meaning 'Italian outline') is a fortification in a style developed during the early modern period in response to the ascendancy of gunpowder weapons such as cannon, which rendered earlier medieval approaches to fortification obsolete. It appeared in the mid-fifteenth century in Italy. Some types, especially when combined with ravelins and other outworks, resembled the related star fort of the same era.

The design of the fort is normally a polygon with bastions at the corners of the walls. These outcroppings eliminated protected blind spots, called "dead zones", and allowed fire along the curtain wall from positions protected from direct fire. Many bastion forts also feature cavaliers, which are raised secondary structures based entirely inside the primary structure.

A defensive wall is a fortification usually used to protect a city, town or other settlement from potential aggressors. The walls can range from simple palisades or earthworks to extensive military fortifications such as curtain walls with towers, bastions and gates for access to the city. From ancient to modern times, they were used to enclose settlements. Generally, these are referred to as city walls or town walls, although there were also walls, such as the Great Wall of China, Walls of Benin, Hadrian's Wall, Anastasian Wall, and the Atlantic Wall, which extended far beyond the borders of a city and were used to enclose regions or mark territorial boundaries. In mountainous terrain, defensive walls such as letzis were used in combination with castles to seal valleys from potential attack. Beyond their defensive utility, many walls also had important symbolic functions – representing the status and independence of the communities they embraced.

Existing ancient walls are almost always masonry structures, although brick and timber-built variants are also known. Depending on the topography of the area surrounding the city or the settlement the wall is intended to protect, elements of the terrain such as rivers or coastlines may be incorporated in order to make the wall more effective.

The Bastille (/bæˈstiːl/, French: [bastij] ) was a medieval fortress in Paris, known as the Bastille Saint-Antoine. It played an important role in the internal conflicts of France and for most of its history was used as a state prison by the kings of France. It was stormed by a crowd on 14 July 1789, in the French Revolution, becoming an important symbol for the French Republican movement. It was later demolished and replaced by the Place de la Bastille.

The castle was built to defend the eastern approach to the city from potential English attacks during the Hundred Years' War. Construction was underway by 1357, but the main construction occurred from 1370 onwards, creating a strong fortress with eight towers that protected the strategic gateway of the Porte Saint-Antoine heading out to the east. The innovative design proved influential in both France and England and was widely copied. The Bastille figured prominently in France's domestic conflicts, including the fighting between the rival factions of the Burgundians and the Armagnacs in the 15th century, and the Wars of Religion in the 16th. The fortress was declared a state prison in 1417; this role was expanded further, first under the English occupiers of the 1420s and 1430s, and then under Louis XI in the 1460s. The defences of the Bastille were fortified in response to the Imperial threat during the 1550s, with a bastion constructed to the east of the fortress. The Bastille played a key role in the rebellion of the Fronde and the Battle of the Faubourg St Antoine, which was fought beneath its walls in 1652.

A curtain wall is a defensive wall between fortified towers or bastions of a castle, fortress, or town.

A ravelin is a triangular fortification or detached outwork, located in front of the innerworks of a fortress (the curtain walls and bastions). Originally called a demi-lune, after the lunette, the ravelin is placed outside a castle and opposite a fortification curtain wall.

The ravelin is the oldest and at the same time the most important outer work of the bastion fortification system. It originated from small forts that were supposed to cover the bridge that led across the moat to the city or fortress gate from a direct attack. From this original function, to protect the gate bridge, also comes its original Italian name "rivellino" (which means small bank work or with the German expression common for it: Brückenkopf – "bridge head"). Therefore, the ravelin was at first only a small work, which should only make the access to the bridge in front of the fortress gates more difficult. When it was realized in the 16th century that this would generally provide better protection for the courtine, ravelins were also built in front of other courtines and these were gradually enlarged. However, it was not until the German fortress builder Daniel Specklin (1536–1589) recognized the principal importance of ravelins (which he still called "ledige Wehr" or "revelin"). He demanded that they be made as large as possible so that they fully covered the courtine and the flanks of the bastions and could place a flanking fire in front of the bastion tops. In the following period, ravelins can be found in practically all fortresses built according to the bastion fortification system.

A cavalier is a fortification which is built within a larger fortification, and which is higher than the rest of the work. It usually consists of a raised platform within a fort or bastion, so as to be able to fire over the main parapet without interfering with the fire of the latter. Through the use of cavaliers, a greater volume of fire can be obtained, but its greater height also makes it an easier target for a besieger's guns.

The siege of Ochakov (Russian: осада Очакова) or the siege of Özi (Turkish: Özi Kuşatması), now Ochakiv, Ukraine, was one of the major events of the Russo-Turkish War (1787–1792). The Ochakov Fortress had the appearance of an irregular quadrangle, consisting of low bastions with a dry moat and glacis on the dry side, and a simple stone wall on the sea side. In addition, 10 advanced lunettes built later extended from the dry side. From the sea, fort Hasan Paşa reinforced the defense. By the time the Russians began the siege, the fortress was a strong fence, but was not able to withstand an active, energetic storming for long. Ochakov was not what it was in the old days, under Münnich in 1737, but it still did not represent an impregnable stronghold that required enormous preparations and expenditure of time. Besieging in summer and autumn, with the arrival of winter, the supreme commander Potemkin decided to stop the siege works and begin the storming of Ochakov (Russian: штурм Очакова) on 17 December [O.S. 6 December] 1788, which ended in Russian success and capture of Hüseyin Pasha, the chief of garrison.

In 1788, Russian forces led by the commander in chief Prince Grigory Potemkin, General Alexander Suvorov, who was called here from near Kinburn Spit with his Phanagorian Regiment and commanded the left wing (until injury and replacement), Prince Nikolai Repnin (the center) and Ivan Möller (the right wing) besieged the city, held by Ottoman troops massively assisted at sea by Hasan Pasha, whose knowledge exceeded the Turkish military level of that time, and commanded by Hüseyin Pasha. Despite Suvorov's urging to storm the city immediately, Potemkin had the Russian forces encircle Ochakov (Özi), bombarding the city and cutting off the defenders' supply of food and ammunition. By keeping his soldiers out of direct battle, Potemkin strove to minimize Russian combat-casualties, though he was accused by his generals, including Suvorov, of cowardice, and the Turks organized several sorties from the fortress and in everyway interfered with the siege at sea. The argument within the Russian headquarters about storming Ochakov continued during the entirety of the siege. The rowing flotilla was commanded by Prince Karl (Charles) Nassau-Siegen, the sailing fleet by Paul Jones, a fighter for American independence; the latter stood in subordination of the first. There was no great agreement between them. Nassau and Jones were subordinate to Potemkin.

The fortifications of Kotor (Italian: Cattaro) are an integrated historical fortification system that protected the medieval town of Kotor containing ramparts, towers, citadels, gates, bastions, forts, cisterns, a castle, and ancillary buildings and structures. They incorporate military architecture of Illyria, the Byzantine Empire, Venice, and Austria. Together with the old town and its natural surroundings the fortifications were inscribed in the list of World Heritage Sites in 1979 labelled Natural and Culturo-Historical Region of Kotor and represent the only such site of cultural significance in Montenegro.

The fortified city of Kotor was also included in UNESCO's World Heritage Site list as part of Venetian Works of Defence between 15th and 17th centuries: Stato da Terra – western Stato da Mar in 2017.

Fort Beauséjour (French pronunciation: [fɔʁ boseʒuʁ]), renamed Fort Cumberland in 1755, is a large, five-bastioned fort on the Isthmus of Chignecto in eastern Canada, a neck of land connecting the present-day province of New Brunswick with that of Nova Scotia. The site was strategically important in Acadia, a French colony that included primarily the Maritimes, the eastern part of Quebec, and northern Maine of the later United States. The fort was built by the French from 1751 to 1752. They surrendered it to the British in 1755 after their defeat in the Battle of Fort Beauséjour, during the Seven Years' War. The British renamed the structure as Fort Cumberland. The fort was strategically important throughout the Anglo-French rivalry of 1749–63, known as the French and Indian Wars by British colonists. Less than a generation later, it was the site of the 1776 Battle of Fort Cumberland, when the British forces repulsed sympathisers of the American Revolution.

Since 1920 the site has been designated as a National Historic Site of Canada, named the Fort Beauséjour – Fort Cumberland National Historic Site. Portions of the fort have been restored, and a museum and visitor facilities were added to the site. It attracts about 6000 visitors annually.

An outwork is a minor fortification built or established outside the principal fortification limits, detached or semidetached. Outworks such as ravelins, lunettes (demilunes), flèches and caponiers to shield bastions and fortification curtains from direct battery were developed in the 16th century. Later, the increasing scale of warfare and the greater resources available to the besieger accelerated this development, and systems of outworks grew increasingly elaborate and sprawling as a means of slowing the attacker's progress and making it more costly. When taken by an enemy force, their lack of rear-facing ramparts left them totally open to fire from the main works.

An advanced work, a fortification detached and forward of the main castle or fortification, is sometimes referred to as a type of outwork or 'advanced outwork'.