Citadel in the context of Hammadids

The Acropolis of Athens (Ancient Greek: ἡ Ἀκρόπολις τῶν Ἀθηνῶν, romanized: hē Akropolis tōn Athēnōn; Modern Greek: Ακρόπολη Αθηνών, romanized: Akrópoli Athinón) is an ancient citadel located on a rocky outcrop above the city of Athens, Greece, and contains the remains of several ancient buildings of great architectural and historical significance, the most famous being the Parthenon. The word Acropolis is from Greek ἄκρον (akron) 'highest point, extremity' and πόλις (polis) 'city'. The term acropolis is generic and there are many other acropoleis in Greece. During ancient times the Acropolis of Athens was also more properly known as Cecropia, after the legendary serpent-man Cecrops, the supposed first Athenian king.

While there is evidence that the hill was inhabited as early as the 4th millennium BC, it was Pericles (c. 495–429 BC) in the fifth century BC who coordinated the construction of the buildings whose present remains are the site's most important ones, including the Parthenon, the Propylaea, the Erechtheion and the Temple of Athena Nike. The Parthenon and the other buildings were seriously damaged during the 1687 siege by the Venetians during the Morean War when gunpowder being stored by the Turkish rulers in the Parthenon was hit by a Venetian bombardment and exploded.

An acropolis was the settlement of an upper part of an ancient Greek city, especially a citadel, and frequently a hill with precipitous sides, mainly chosen for purposes of defense. The term is typically used to refer to the Acropolis of Athens, yet nearly every Greek city had an acropolis of its own. Acropolises were used as religious centers and places of worship, forts, and places in which the royal and high-status resided. Acropolises became the nuclei of large cities of classical ancient times, and served as important centers of a community. Some well-known acropolises have become the centers of tourism in the present day, and they are a rich source of archaeological information of ancient Greece, especially, the Acropolis of Athens.

Mycenae (/maɪˈsiːniː/ my-SEE-nee; Ancient Greek: Μυκῆναι or Μυκήνη, Mykē̂nai or Mykḗnē) is an archaeological site near Mykines in Argolis, north-eastern Peloponnese, Greece. It is located about 120 kilometres (75 miles) south-west of Athens; 11 kilometres (7 miles) north of Argos; and 48 kilometres (30 miles) south of Corinth. The site is 19 kilometres (12 miles) inland from the Saronic Gulf and built upon a hill rising 274 metres (899 feet) above sea level.

In the second millennium BC, Mycenae was one of the major centres of Greek civilization—a military stronghold which dominated much of southern Greece, Crete, the Cyclades and parts of southwest Anatolia. The period of Greek history from about 1600 BC to about 1100 BC is called Mycenaean in reference to Mycenae. At its peak in 1350 BC, the citadel and lower town had a population of 30,000 and an area of 32 hectares (79 acres).

Arx is a Latin word meaning "citadel". In the ancient city of Rome, the arx was located on the northern spur of the Capitoline Hill, and is sometimes specified as the Arx Capitolina.

In Greek and Roman mythology, the Palladium or Palladion (Greek Παλλάδιον (Palladion), Latin Palladium) was a cult image of great antiquity on which the safety of Troy and later Rome was said to depend, the wooden statue (xoanon) of Pallas Athena that Odysseus and Diomedes stole from the citadel of Troy and which was later taken to the future site of Rome by Aeneas. The Roman story is related in Virgil's Aeneid and other works. Rome possessed an object regarded as the actual Palladium for several centuries; it was in the care of the Vestal Virgins for nearly all this time.

Since around 1600, the word palladium has been used figuratively to mean anything believed to provide protection or safety, and in particular in Christian contexts a sacred relic or icon believed to have a protective role in military contexts for a whole city, people or nation. Such beliefs first become prominent in the Eastern church in the period after the reign of the Byzantine Emperor Justinian I, and later spread to the Western church. Palladia were carried in procession around the walls of besieged cities and sometimes carried into battle.

The Tower of London, officially His Majesty's Royal Palace and Fortress of the Tower of London, is a historic citadel and castle on the north bank of the River Thames in London, England. It lies within the London Borough of Tower Hamlets, which is separated from the eastern edge of the square mile of the City of London by the open space known as Tower Hill. It was founded toward the end of 1066 as part of the Norman Conquest. The White Tower, which gives the entire castle its name, was built by William the Conqueror in 1078 and was initially a resented symbol of oppression, inflicted upon London by the new Norman ruling class. The castle was also used as a prison from 1100 (Ranulf Flambard, Bishop of Durham) until 1952 (the Kray twins), although that was not its primary purpose. A grand palace early in its history, it served as a royal residence. As a whole, the Tower is a complex of several buildings set within two concentric rings of defensive walls and a moat. There were several phases of expansion, mainly under kings Richard I, Henry III, and Edward I in the 12th and 13th centuries. The general layout established by the late 13th century remains despite later activity on the site.

The Tower of London has played a prominent role in English history. It was besieged several times, and controlling it has been important to controlling the country. The Tower has served variously as an armoury, a treasury, a menagerie, the home of the Royal Mint, a public record office, and the home of the Crown Jewels of England. From the early 14th century until the reign of Charles II in the 17th century, the monarch would traditionally prepare for several nights at the Tower, and lead a procession from there to Westminster Abbey for their coronation. In the absence of the monarch, the Constable of the Tower was in charge of the castle. This was a powerful and trusted position in the medieval period. In the late 15th century, the Princes in the Tower were housed at the castle when they mysteriously disappeared, presumed murdered. Under the Tudors, the Tower was used less as a royal residence, and despite attempts to refortify and repair the castle, its defences lagged behind developments to deal with artillery.

Castillo San Felipe del Morro (English: Promontory Castle of Saint Philip), most commonly known as El Morro (The Promontory), is a large fortress and citadel in the Old San Juan historic quarter of San Juan, the capital municipality of Puerto Rico. Commissioned by King Charles I of Spain in 1539, it was first built as a fortified tower in honor of King Philip II, who oversaw its expansion into a hornwork bastion fort by 1595. Over the next 200 years, especially in the reign of King Charles III, El Morro continued to be developed to reach its current form in 1787. Rising 43 metres (140 ft) from the Atlantic shoreline with 5.5 to 7.6 metres (18 to 25 ft) thick walls, the six-leveled edifice stands on a steep, rocky headland promontory on San Juan Islet guarding the entry to San Juan Bay, the harbor of Old San Juan. El Morro, alongside La Fortaleza, San Cristóbal, El Cañuelo, and other forts part of the Walls of Old San Juan, protected strategically and militarily important Puerto Rico, or La Llave de las Indias (The Key to the Indies), from invasion by competing world powers during the Age of Discovery and Sail. It was designated a World Heritage Site by UNESCO in 1983.

Ajaccio is the capital and largest city of Corsica, France. It forms a French commune, prefecture of the department of Corse-du-Sud, and head office of the Collectivité territoriale de Corse (capital city of Corsica). It is also the largest settlement on the island. Ajaccio is located on the west coast of the island of Corsica, 210 nautical miles (390 km) southeast of Marseille.

The original city went into decline in the Middle Ages, but began to prosper again after the Genoese built a citadel in 1492, to the south of the earlier settlement. After the Corsican Republic was declared in 1755, the Genoese continued to hold several citadels, including Ajaccio, until the French took control of the island.

The Cadmea, or Cadmeia (Greek: Καδμεία, Kadmía), was the citadel of ancient Thebes, Greece, which was named after Cadmus, the legendary founder of Thebes. The area is thought to have been settled since at least the early Bronze Age, although the history of settlement can only be reliably dated from the late Mycenaean period (c. 1400 BC).

The Belgrade Fortress (Serbian Cyrillic: Београдска тврђава, romanized: Beogradska tvrđava, Hungarian: Nándorfehérvár), consists of the old citadel (Upper and Lower Town) and Kalemegdan Park (Large and Little Kalemegdan) on the confluence of the Sava and Danube rivers, in an urban area of modern Belgrade, Serbia. Located in Belgrade's municipality of Stari Grad, the fortress constitutes the specific historical core of the city. As one of the most important representatives of Belgrade's cultural heritage, it was originally protected right after World War II, among the first officially declared cultural monuments in Serbia. The fortress was declared a Monument of Culture of Exceptional Importance in 1979, and is protected by the Republic of Serbia. It is the most visited tourist attraction in Belgrade, with Skadarlija being the second. Since the admission is free, it is estimated that the total number of visitors (foreign, domestic, citizens of Belgrade) is over 2 million yearly.

Visegrád (Hungarian: [ˈviʃɛɡraːd]; German: Plintenburg; Latin: Pone Navata or Altum Castrum; Slovak: Vyšehrad) is a castle town in Pest County, Hungary. It is north of Budapest on the right bank of the Danube in the Danube Bend. It had a population of 1,864 in 2010. The town is the site of the remains of the Early Renaissance summer palace of King Matthias Corvinus of Hungary and a medieval citadel. The town was also the site of a meeting on 15 February 1991 between the President of the Czech and Slovak Federative Republic, Václav Havel, the President of the Republic of Poland, Lech Wałęsa, and the Prime Minister of the Republic of Hungary, József Antall which founded the titular Visegrád group.

A palladium or palladion (plural palladia) is an image or other object of great antiquity on which the safety of a city or nation is said to depend. The word is a generalization from the name of the original Trojan Palladium, a wooden statue (xoanon) of Pallas Athena that Odysseus and Diomedes stole from the citadel of Troy. It was supposedly later taken to the future site of Rome by Aeneas, where it remained until perhaps transferred to Constantinople and was lost after the Empire converted to Christianity.

In English, since around 1600, the word "palladium" has been used figuratively to mean anything believed to provide protection or safety, and in particular in Christian contexts a sacred relic or icon believed to have a protective role in military contexts for a whole city, people or nation. Such beliefs first become prominent in the Eastern Churches in the period after the reign of the Byzantine Emperor Justinian I, and later spread to the Western church. Palladia were processed around the walls of besieged cities and sometimes carried into battle. In this more offensive role they may also be referred to as "vexilla" (singular vexillum, Latin for "battle standard").

Petersberg Citadel (German: Zitadelle Petersberg) in Erfurt, central Germany, is one of the largestand best-preserved town fortresses in Europe. The citadel was built on Petersberg hill, in the north-western part of the old town centre from 1665, when Erfurt was governed by the Electorate of Mainz. It is surrounded by over two kilometres of stone walls and is 36 hectares in size.

Erfurt has also been ruled by Sweden, Prussia, Napoleon, the German Empire, the Nazis, and post-World War II Soviet occupying forces, and it was part of the German Democratic Republic (East Germany). All of these regimes used Petersberg Citadel and had an influence on its development. The baroque fortress was in military use until 1963. Since German reunification in 1990, the citadel has undergone significant restoration and it is now open to the public as a historic site. Its maze of underground passageways are also open to visitors and the fortress bakery (1832) is again in working order.

The Lion Gate (Greek: Πύλη των Λεόντων) is the popular modern name for the main entrance of the Bronze Age citadel of Mycenae in Southern Greece. It was erected during the thirteenth century BC, around 1250 BC, in the northwestern side of the acropolis. In modern times, it was named after the relief sculpture of two lions or lionesses in a heraldic pose that stands above the entrance.

The gate is the sole surviving monumental piece of Mycenaean sculpture, as well as the largest surviving sculpture in the Bronze Age Aegean. It is the only monument of Bronze Age Greece to bear an iconographic motif that survived without being buried underground. It is the only relief image that was described in the literature of classical antiquity, such that it was well known prior to modern archaeology.

Dendra (Greek: Δενδρά) is a prehistoric archaeological site situated outside the village with the same name belonging to the municipality of Midea in the Argolid, Greece.

The site was inhabited during the Neolithic and Early Helladic periods, and is known for its Late Bronze Age cemetery. In the first half of the 20th century, the Swedish archaeologist Axel W. Persson excavated an unplundered tholos tomb and many Mycenaean chamber tombs, presumably belonging to the ruling classes of the nearby citadel of Midea. The finds from the site include the Dendra panoply, a set of Mycenaean bronze armour found in one of its chamber tombs, and the Dendra Octopus Cup from the tholos. From the Hellenistic into the Byzantine periods, it was the site of a village: local tradition recalled that it had once been known as "Sanga".

Machu Picchu is a 15th-century Inca citadel located in the Eastern Cordillera of southern Peru on a mountain ridge at 2,430 meters (7,970 ft). It is situated in the Machupicchu District of Urubamba Province about 80 kilometers (50 miles) northwest of Cusco, above the Sacred Valley and along the Urubamba River, which forms a deep canyon with a subtropical mountain climate.

Often referred to as the "Lost City of the Incas", Machu Picchu is one of the most iconic symbols of the Inca civilization and a major archaeological site in the Americas. Built around 1450, it is believed to have served as an estate for the Inca emperor Pachacuti, though no contemporary written records exist to confirm this. The site was abandoned roughly a century later, likely during the Spanish conquest. Modern radiocarbon dating places its occupation between c. 1420 and 1530.