The ruins of ancient Eretria are dispersed beneath the modern city and extend from the shore to the hill in the north, where the acropolis once stood. A fortification wall built in the Archaic period and repaired in the fourth century BC protected the city. The wall, approximately four kilometres long, began on the acropolis and extended down to the port forming a rough rectangle. Its west section lined the rivulet that marked the western city limits and projected into the sea forming a jetty, now preserved underwater. The wall's east section also projected into the sea forming a natural jetty. These east and west jetties were connected by a coastal fortification wall. The two main city gates were located on the wall's west and east sections. The west gate, the city's earliest and most important gate, was built over the rivulet together with a bridge.

Within the wall were the three city quarters (east, west, and north), separated form one another by two main roads. The first road connected the east and west gates, dividing the city longitudinally; the second road began at approximately the middle of the first road and crossed the city in a north-south direction, leading to the temple of Apollo Daphnephoros, which dominated the city centre. South of the temple was the ancient agora with its stoas, shops, tholos (an impressive circular building), springs, and temples. Along the south coastal fortification wall are bathhouses of the third century BC, workshops, warehouses, and dockyards. Finally, the temple of Isis and the lower gymnasium or palaestra, both buildings of the fourth century BC, repaired after 198 BC, are located near the port, in the south-west corner of the city.

The city's west quarter developed at the foot of the acropolis' hill, next to the west gate, during the Geometric, Classical, and Hellenistic periods. A number of grand buildings, including a heroon with rich burials of the Geometric period, Palaces I and II, several grand residences of the fourth century BC, and a temple of the fifth century BC survive along the west wall, south of the gate. The temple of Dionysus and the theatre, Eretria's most impressive monument, built in the fifth century and completed in the fourth century BC, stand north of the gate. East of the theatre is the upper gymnasium, built in the Late Classical period and repaired after 198 BC, and nearby was the stadium, whose location is known from inscriptions and travelers' descriptions. The Thesmophorion, or temple of Demeter Thesmophoros, built in the third century BC, and the temple of Artemis, an earlier building, which remained in use until the third century BC, stood on the slope of the hill further north.

The remains of the famous House of the Mosaics can be seen near the intersection of the two main streets. This well-preserved house, a typical example of Eretrian architecture, dates from the fourth century BC. Buildings of the Geometric and Hellenistic periods, and part of a circular structure with an altar, probably dedicated to Hercules, survive a little further south near the city centre. Part of a fortification wall used for a short time period (800-700 BC) to define the north and northwest limits of the settlement stands in the same area. This quarter was also inhabited in the Hellenistic period, along with the west quarter, and until the Roman period.

Eretria's two cemeteries were located outside the city walls, the Archaic cemetery to the west and the later one to the east. The burials (cremations and inhumations) were placed inside stone or marble sarcophagi and tile-covered graves.