Excavations at Kos, initiated systematically in the early 20th century CE and continued after the 1933 earthquake that exposed extensive ruins, have revealed a cityscape of remarkable organization and cultural synthesis. The archaeological record shows a densely planned urban center with monumental public buildings, including the agora and harbor sanctuaries, surrounded by well-developed residential quarters. The Asclepeion—its most emblematic complex—exemplifies the union of religious devotion and scientific medicine that made Kos a center of healing across the Hellenistic and Roman worlds. Elsewhere, the remains of the Roman Odeon, the Casa Romana, and several bath complexes attest to social and artistic life, while defensive works and later Christian basilicas trace the island’s continuity and transformation through subsequent centuries. Together, these structures illustrate not only the material sophistication of the Koan polis but also its role as a regional hub where architecture served civic order, economic exchange, and the expression of collective identity.

Their layout reflects the Hippodamian planning principles adopted after the synoecism of circa 366 BCE, when the capital was moved and rebuilt as a unified polis. Within the context of Aegean archaeology, the remains of Kos constitute a coherent corpus of Hellenistic and Roman architectural practice that demonstrates how an insular Dorian city adapted metropolitan urban models to local topography, economic life, and cultic traditions.

circa 400 BCE

Ancient Agora
The Ancient Agora of Kos, dating to the 4th century BCE, is a multi-layered archaeological site situated in the eastern sector of the modern city, near the harbor. Its historical significance stems from its role as the administrative, commercial, and social heart of Kos following the island's capital relocation in 366 BCE. The visible ruins, largely a result of extensive Italian-led excavations following the devastating 1933 earthquake, reveal a complex stratigraphy representing different construction phases. The original 4th-century construction, utilizing limestone and travertine, gave way to more monumental marble edifices in the 2nd century BCE.

The site, one of the largest excavated agoras in Greece, encompasses not only the central market but also the foundations of sanctuaries dedicated to deities such as Aphrodite and Heracles, a basilica, and sections of the ancient city walls. Significant evidence of Roman-era reconstruction following the 142 CE earthquake and the definitive end of the ancient city after the 469 CE seismic event highlight the island's resilience and subsequent repurposing of its urban space in the Early Christian period. The surviving remnants, including columns, stoa foundations, and mosaic floors, offer critical insights into the urban planning, economic activities, and religious practices of ancient Kos.

circa 400 BCE

Asklepieion Complex
The Asklepieion complex on Kos is a multi-layered archaeological site that developed over several centuries, beginning in the fourth century BCE on a sacred cypress grove dedicated to Apollo and Asclepius. Its tiered construction across three major terraces is a defining feature, designed to maximize the therapeutic potential of the natural landscape. The lowest terrace housed treatment rooms, Roman-era baths, and the impressive Propylon entrance. The middle terrace, accessible via a monumental staircase, was the spiritual and intellectual heart of the complex, containing the oldest altar to Asclepius (mid-fourth century BCE), the priestly residence (abaton), and a third-century BCE Temple of Asclepius.

At the highest level, a large Doric temple dedicated to Asclepius, dating to the second century BCE, dominated the sanctuary, visible from the harbor and demonstrating its panhellenic significance. After its decline due to earthquakes, particularly in the 6th century CE, the complex's stones were extensively repurposed by the Knights Hospitaller for Neratzia Castle, while the site itself was partially occupied by an early Christian church. Excavations by German and Italian archaeologists in the early 20th century CE uncovered the complex, revealing invaluable insights into the synthesis of religious worship, medical practice, and architectural design in ancient Kos.

circa 400 BCE

East Stoa
The east stoa of the agora, 316 meters in length, retains its course and form unaltered throughout all the building phases of the ancient monument. Along its north side, in the the harbour, situated at the rear part of the portico, there were oblong spaces that apparently served as storage facilities of trade goods. In the fourth century BCE their entrance was sealed with metal panels, which were replaced with doors in the second century BCE. During the Imperial Roman times, when commercial activity moved out of the agora, they were widened and divided into three aisles, serving obviously, different purposes.

On the contrary, in the south part of the agora— the spaces of the east stoa maintain the same layout in all of the building phases: they nave a bipartite ground plan, with the east portion further divided into two parts. They probably operated as workshops (i.e. for the production of colour pigments using litharge as raw material), while some of them were also shops.

In a land property adjacent to the area of the altar of Dionysos and the temple of the Attalids, excavation uncovered poor remains from the east stoa, at a depth of 3.5 meters below ground level, now reburied in situ. Parts of the colonnade were revealed, as also two shops and the street, aligned east-west, that proviaed direct access from the secular domestic core of the city to the public space of the agora.

The expropriation of the property by the state opened up the opportunity to create a space for recreation and educational programmes focusing on the ancient agora, the layout which is easily perceived through the simulation of the east stoa by features built of new materials at 1:1 scale at the ground level of the archaeological site. These features render the stylobate with the lower part of the Doric columns, the floor of the stoa, the shops and the street aligned east-west.

circa 323-146 BCE

Ancient Theater of Astypalaia
The Ancient Theater of Astypalaia, located at the site of Palatia near modern Kefalos on the island of Kos, is a Hellenistic-period structure dating to the fourth century BCE. Built during the period when Astypalaia served as the island's capital, the theater's design exemplifies classical Greek architectural principles by utilizing the natural slope of the terrain for its koilon (seating area). Excavated by the Italian Archaeological Mission in 1928 CE, the site retains notable structural elements, including the foundation of its 13-meter stage, the bases of four Doric columns, and the circular orchestra with its central thymele (altar) dedicated to Dionysus. The monument provides valuable insight into the cultural and administrative prominence of ancient Astypalaia, which was a member of the First Athenian League before the capital was relocated to Kos Town in 366 BCE. While only a few rows of the original seating survive, the extant ruins, situated with panoramic views overlooking the Kefalos bay, represent a significant archaeological feature of the ancient city.

circa 323-146 BCE

Sanctuary of Hercules
The attribution of the sanctuary to Herakles is based on epigraphic testimonies and part of a lion-skin from the colossal statue of the mythical hero, presently built into the wall of the Loggia mosque, which were found in the vicinity of the sanctuary. The sanctuary was founded in the late fourth to early third century BCE on a trapezoidal area created for this purpose by relocating the harbour wall further east. A temple was possibly constructed on the site, but no remains have been identified. A stoa on the north side served as the monumental propylon of the sanctuary. In the early second century BCE the temple was built in the form of a simple oikos upon a podium. Access was via a flight of ten steps. A stoa, small parts of the foundation of which are preserved, was added to the west side.

In the first to second century CE, porticoes with rooms were added to the north, south and east sides of the court. After the catastrophic earthquake in 142 CE, repairs were made mainly to the ancillary rooms. Mosaic floors of exquisite art, with representations referring to the cult of Herakles, were laid in some of these. The sanctuary was destroyed by the earthquake of 469 CE, by which period it had perhaps already lost its religious status. Thermae were built upon its ruins, continuing in existence until the seventh century CE. During the Hospitaller period the church of Saint Demetrios was erected on the remnants of the temple. It was demolished in order to uncover the sanctuary in its entirety.

The mythical hero Herakles and his companions were driven ashore by a tempest in the area of Cape Laketeras, Kardamaina, whilst homeward bound after the labour of seizing the girdle of the Amazon Hppolyta. There they clashed with the renowned Koan athlete Antagoras, when they took refuge in his sheepfold, in search of food. The inhabitants of Kos, the Meropes, ran for help and forced Herakles and his companions to flee. In order to protect himself, the hero dressed in women's clothes. Immediately afterwards, having rallied round, they succeeded in counteracting the Meropes. After the victory, Herakles, weanng female attire, married Chalkiope, daughter of King Eurypylos of Kos. She bore him a son, Thessalos, father of Antiphos and Pheidippos, who commanded the 10 ships that Kos, Nisyros, the Calydnian islands and Kasos sent to the Trojan War (Iliad II, 676-679 CE).

circa 220-180 BCE

Sanctuary of Aphrodite
The Sanctuary of Aphrodite, located within the Eastern Archaeological Area of Kos Town, is an important example of a Hellenistic religious structure that was later built upon during the Roman and Early Christian eras. Constructed on a high, artificial podium in the late third or early second century BCE, the sanctuary's core comprised a large, colonnaded courtyard, within which stood two identical prostyle temples with altars, dedicated to Aphrodite Pandemos and Aphrodite Pontia. The strategic placement of the sanctuary, facing the harbor entrance, underscores its significance for maritime activities on Kos. Following its destruction by an earthquake in 469 CE, the site entered a phase of repurposing, with building materials from the original sanctuary being integrated into a new basilica constructed during the fifth and sixth centuries. The current archaeological state thus reflects a complex history of construction, destruction, and reuse, revealing the successive cultural layers that define the ancient city's port district.

circa 180 BCE

Temple of the Attalids
The temple of the Ättalids, aligned North-South, is dated to the first half of the second century BCE and was in use until the Roman Empire period, when large-scale remodelling was carried out in this part of the ancient agora. From the Hellenistic temple only the foundation of the stepped podium is preserved, built of ignimbrite, rock, along with sporadic parts from its facade of hard grey limestone. Although no architectural parts have been found, it is reconstructed by scholars as a distyle in-antis temple of Ionic order or a tetrastyre prostyle temple of Doric order.

Around the temple, stone-built pedestals are preserved, meant probably for votive offerings, while in its vicinity part of a colossal statue was found, depicting a thorakophoros (male figure wearing a cuirass) with his himation wrapped around his waist. Carved in relief on the strut of the statue are two shields of Gallic type. The thorakophoros is identified with king Eumenes II of Pergamon (197-158 BCE), who was honoured by the Koans On the occasion of his first victory against the Gauls in 183 BCE, The organized cult of Eumenes in Kos, also known from epigraphic evidence, is attributed to the particularly close relations the two states established in that period. It is thought that a statue of the victorious king had been erected in the pronaos (anteroom) of the temple and that he was worshipped as synnaos (temple-sharing) with another deity, probably Dionysos, founder of the House of the Attalids.

circa 150 BCE

Altar of Dionysus
The Altar of Dionysus is a Hellenistic archaeological site situated in Kos Town, Greece, near the ancient Agora. The structure, constructed during the 2nd century BCE, was dedicated to Dionysus, the god of wine, ritual madness, and fertility. Excavated by the Italian Archaeological Mission in the early 1930s, the altar is characterized by its Π-shaped plan and was built from white and grey marble.

Archaeological findings indicate that the altar's exterior was once adorned with a significant relief frieze. This frieze depicted scenes from Dionysian mythology, including an Amazonomachy and a thiasus, a procession of Dionysus's followers such as satyrs, sileni, and maenads. After an earthquake in 142 CE, the altar sustained considerable damage. In the 15th century CE, the Knights of Saint John incorporated sections of the frieze into the masonry of their fortress, Neratzia Castle, to fortify the structure. Today, fragments of the ornate frieze have been salvaged and are displayed within the castle's museum.

The visible remains at the site include the rectangular foundation (crepidoma), part of the main entrance, and a stone wall. An adjacent Doric temple, possibly also dedicated to Dionysus, is part of the archaeological complex. The site is positioned approximately three meters below the level of the modern city, illustrating the historical layering of Kos town. It serves as a key testament to the island's Hellenistic-era prosperity and religious practices.

circa 1436-1514 CE

Neratzia Castle of Knights Hospitaller
Neratzia Castle in Kos Town is a striking archaeological and historical structure, built by the Knights Hospitaller between the 14th and early 16th centuries. Its construction occurred in phases, with the inner enclosure predating the outer, which was heavily fortified with massive bastions and artillery ports after 1495 to counter advanced gunpowder weaponry. The castle's most distinguishing archaeological feature is its extensive reuse of building materials, including columns, inscriptions, and architectural fragments from ancient Kos, particularly the Asklepieion. This practice is evident throughout the structure, where Hellenistic friezes with sculpted masks and garlands are integrated into medieval walls. Located strategically on a former islet at the harbor entrance, Nerantzia Castle controlled the sea passage and served as a barracks, residence for the Ottoman commander, and later a German prison. Despite suffering damage from an 1816 gunpowder explosion, the castle's restored form, shaped by Italian archaeological efforts in the early 20th century, remains a tangible record of the island's successive historical occupations.

circa 1322-1346 BCE

Antimachia Castle
Antimachia Castle on Kos is a significant example of medieval military architecture, reflecting its construction by the Knights Hospitaller of Saint John. Its history is marked by strategic relevance, serving as a prison for wayward knights and a central refuge for local populations during sieges. The castle's layout demonstrates a keen sense of defensive strategy, utilizing a hilltop location for panoramic views and a design that incorporates the natural terrain. While much of the original settlement within the walls was lost, archaeological investigation has revealed multiple construction phases dating from the 14th to the 16th century, along with additions during the subsequent Ottoman occupation. Key structural remnants include the impressive defensive walls, a massive crescent-shaped bastion at the entrance from the early 16th century, and cisterns for water supply. Additionally, two notable churches—the 16th-century Byzantine Agios Nikolaos and the 18th-century chapel of Agia Paraskevi—have survived, indicating the site's continued use long after its initial military purpose faded.