Roman Bulgaria occupied an important position in the heart of the Balkans, between the Black Sea, the Aegean, and Asia Minor. In the north, the Danube River formed a heavily defended military frontier, lined with auxiliary camps and fortresses. Further south were the cities of Thrace, while older Greek settlements spread along the Black Sea coast. The Roman remains preserved in Bulgaria today are remarkably varied and include theatres, stadiums, baths, fortifications, early Christian churches and some exceptional mosaic floors.

Before the Roman conquest, much of what is now Bulgaria was originally inhabited by Thracians, while Greek cities had already existed along the Black Sea coast for centuries. Roman involvement with the region began during the Roman Republic, mostly along the Via Egnatia, but control developed gradually, reaching the Lower Danube by the early 1st century CE. Soon afterwards, the province of Moesia was created and was later divided into Moesia Superior and Moesia Inferior. South of the Balkan Mountains, the Thracian kingdom remained a client state of Rome until 45 to 46 CE, when Claudius (reign 41 to 54 CE) annexed it and established the province of Thrace.

The Roman Empire maintained a strong military presence along the Danube, where legions and auxiliary units occupied forts and garrisons while settlements grew around them. Four legions were stationed on the frontier, including the V Macedonica (at Oescus), IV Scythica, VIII Augusta (at Novae), and XI Claudia (at Durostorum). Thrace, however, was much less heavily militarised, and Greek remained the main language of its cities. The construction of roads along the south bank of the Danube provided logistical support for the Roman army and facilitated rapid troop movements, while others connected the Danube with the Black Sea, the Aegean and Byzantium. One of the most important roads was the Via Diagonalis (or Via Militaris), which crossed Thrace. Towns situated on these routes could prosper, and smaller commercial centres (emporia) also appeared along them. Philippopolis, Serdica, Augusta Traiana, Nicopolis ad Istrum and Oescus were among the principal cities.

The 2nd century CE saw extensive building work, including temples, theatres, Roman baths and aqueducts, while new cities were founded. The most extensive building activity occurred during the reign of Commodus (180 to 192 CE) and the Severan dynasty (193 to 235 CE), while the Costobocae incursion around 170 CE prompted several cities to reinforce their defences. From the late 230s CE, conditions became much less stable. Goths, Carpi and other groups repeatedly crossed the Lower Danube, bringing warfare into the provinces. Philippopolis was besieged by the Goths in 250 CE, and the following year Emperor Decius (reign 249 to 251 CE) was killed at the Battle of Abritus. Aurelian's withdrawal from Dacia in 270 to 272 CE made the Danube the main frontier once again. Diocletian (reign 284 to 305 CE) and Constantine I (reign 306 to 337 CE) later divided the region into several provinces grouped within the Thracian diocese.

The foundation of Constantinople in 330 CE brought the region much closer to the imperial centre. Christianity also became increasingly visible, especially in the large basilicas built in many cities. The Balkans remained within the Eastern Roman Empire after Roman rule ended in the West. Justinian I (reign 527 to 565 CE) repaired and strengthened numerous fortifications, but renewed invasions in the 6th and 7th centuries contributed to the decline or abandonment of many Roman cities.

Here are ten Roman sites particularly worth visiting.

Plovdiv has to come first on this list. It is a very pleasant city to explore, with the remains of Roman Philippopolis appearing throughout the modern centre. The settlement itself is much older than the Roman conquest. Philip II of Macedon (reign 359 to 336 BCE) captured the Thracian city in the 4th century BCE, and it became known as Philippopolis in his honour. When Rome annexed the Thracian kingdom in 46 CE, the city became part of the province of Thracia.

During the Roman period, Philippopolis gradually extended. Its location on important routes towards the Aegean and Asia Minor contributed to its growth, and by the 1st and 2nd centuries CE the lower city had acquired many of its principal monuments: the theatre, stadium, forum, Odeon, baths and aqueducts. Philippopolis also hosted the provincial assembly of Thrace, whose members gathered here for religious ceremonies and festivals.

The theatre is the most spectacular Roman monument to survive in Plovdiv. Built into the slope between two hills, it offers great views over the Thracian plain and the Rhodope Mountains. Inscriptions on the seating reserved places for magistrates, priests, and representatives of the provincial assembly of Thrace. Excavated and restored in the 20th century, the theatre is once again used for performances.

The forum lay at the centre of the lower city and developed in several phases between the 1st and 4th centuries CE, with colonnaded porticoes and rows of shops surrounding its large, paved square. On the northern side stood the Odeon/Bouleuterion, a library and a treasury, all part of the civic and administrative centre of Philippopolis.

North of the forum stood the enormous Roman stadium, built in the 2nd century CE. About 250 m (820 ft) long, it could hold tens of thousands of spectators. Games were held there in honour of Hadrian's favourite, Antinous, after his death in 130 CE, and under Elagabalus (reign 218 to 222 CE) the Kendrisian Pythian Games celebrated Apollo Kendrisos. Most of the stadium now lies beneath Plovdiv's pedestrian street. Its curved northern end can be seen at Dzhumaya Square, while part of the cavea survives underneath the H&M store.

At the eastern edge of Philippopolis stood an honorary arch dating to Hadrian's reign (117 to 138), probably erected to mark the emperor's visit to the city. When the lower city was fortified under Marcus Aurelius (reign 161 to 180 CE), the arch was incorporated into the new defences. In the 4th century CE, a monumental eastern gate was built on the same site, opening onto the road to Byzantium.

Mosaics are among the highlights of Late Antique Philippopolis. The Bishop's Basilica, first built in the 4th century CE and later enlarged, preserves extensive floors decorated with geometric patterns and numerous birds. Nearby, the 5th-century CE Small Basilica contains another rich group of mosaics, together with a baptistery and cross-shaped baptismal pool. Finds from Roman Philippopolis can also be seen in the Regional Archaeological Museum.

Roman Serdica lies directly beneath the centre of modern Sofia. Founded in the 1st century BCE, the city expanded considerably under Trajan (reign 98 to 117 CE), who granted it municipal status and renamed it Ulpia Serdica. After the Costoboci attack around 170 CE, substantial fortifications were built under the reigns of Marcus Aurelius and Commodus.

Much of Serdica can still be explored at two sites. The first is located under the Largo near Independence Square in central Sofia, where part of the decumanus maximus, the main east-west street, and ancient shop foundations survive. A second archaeological zone beneath Knyaginya Maria Luiza Boulevard preserves remains of streets, baths, two early Christian basilicas, houses and shops.

One of the most interesting buildings is the so-called Felix House, a large residence arranged around a courtyard with a mosaic-floored reception room. The city had an amphitheatre, built towards the end of the 3rd century CE over an earlier Roman theatre. Parts of its arena and seating have also been uncovered beneath modern Sofia.

Serdica became an important city in Late Antiquity. Constantine I spent time here, and considerable building activity took place during his reign. The best-preserved Roman structure is the Church of Saint George, a circular brick building dating to the early 4th century CE. Later converted into a church, it now stands in a courtyard where Roman ruins sit beside modern government buildings.

The city continued to flourish into the 4th-6th centuries. Its defences were strengthened, and churches and large residences occupied much of the area within the walls. The National Archaeological Museum in the city centre is also worth visiting for its Roman sculpture, inscriptions and votive reliefs from Serdica and elsewhere in Bulgaria.

Nicopolis ad Istrum is one of the best places in Bulgaria to appreciate the layout of a Roman city. Unlike Philippopolis or Serdica, it was never covered by a large modern settlement, so its paved streets, forum and public buildings remain easy to follow on the ground.

Trajan founded Nicopolis ad Istrum after his victories over the Dacians in 106 CE. Despite its name, which means "City of Victory on the Danube," Nicopolis was located approximately 50 kilometers (31 miles) south of the river. The city developed as a Greek-dominated administrative centre, with a council (boule) and an assembly (demos). Its strategic position along the routes connecting the Danube, Thrace, and the Black Sea contributed to its prosperity.

The forum formed the heart of Nicopolis. Almost square in plan, it was surrounded by Ionic colonnades and decorated with statues of emperors and local benefactors. Monumental Corinthian propylaea opened onto it from the decumanus maximus. Nearby stood the bouleuterion, a civic basilica and an Odeon seating around 350 spectators.

The streets are among the site's most impressive features. They consist of large paving slabs and provide access to houses and public buildings. Beneath them lies an extensive system of brick-built drains.

Nicopolis was initially an unwalled city, but fortifications were added after the Costoboci attacked around 170 CE. The city recovered under the Severan dynasty, only to face repeated invasions from the 3rd century CE onwards. Its later decline and abandonment help explain why the site is so well preserved and rewarding to visit today.

Near the village of Gigen lie the remains of Ulpia Oescus, one of the most important Roman cities of northern Bulgaria. Its beginnings were military. Legio V Macedonica was stationed here during the 1st century CE, and the future emperor Hadrian served as a military tribune with the legion at Oescus in 96 CE.

After Trajan's Dacian Wars, Legio V Macedonica was transferred to Troesmis and a colony was established on the site. Colonia Ulpia Oescensium soon became a major city in Moesia Inferior, linked by road with other settlements along the Danube and with Serdica and Philippopolis further south.

The excavated city preserves stone-paved streets, a forum, a civic basilica and several temples. The basilica was decorated with an unusual series of caryatids and portrait medallions, and the Temple of Fortuna stood nearby. The site's most beautiful feature, however, is the sheer quantity of architectural fragments scattered across the ground: columns, capitals, cornices and richly carved blocks that once belonged to its monumental buildings.

The most beautiful feature of the site, however, is the abundance of architectural fragments scattered across the ground, left by past invasions and earthquakes. Columns, capitals, cornices, and richly carved blocks that once belonged to the city's monumental buildings now lie on the ground.

One of the most famous finds from Oescus is the Achaeans Mosaic, discovered in 1948 and dating to the late 2nd or early 3rd century CE. It depicts a scene from The Achaeans, a lost Greek comedy by Menander. A Greek inscription identifies the subject as Menandrou Achaioi ("The Achaeans of Menander"). The mosaic is now displayed in the Regional Historical Museum in Pleven.

Oescus returned to prominence under Constantine I. In 328 CE, a great bridge across the Danube linked the city with Sucidava in present-day Romania. It remained occupied into Late Antiquity, and Justinian I restored its fortifications before the city was devastated during the Avar and Slav invasions at the end of the 6th century.

The Roman city of Augusta Traiana lies beneath modern Stara Zagora, in the Upper Thracian Plain. Founded under Trajan around 106 CE and named after the emperor, the city developed into a prosperous administrative and commercial centre.

Part of the Roman city survives in the Antique Forum, an open-air archaeological park. Here visitors can see the western city gate, sections of the fortifications, public baths and part of the decumanus maximus. Beside the paved square stood an unusual semicircular auditorium, with seating for around 1,300 people, used for assemblies and performances.

Augusta Traiana is well known for its floor mosaics, including the famous Procession of Dionysus Mosaic, which decorated a large 4th-century CE residence. Its surviving figurative panel shows members of the thiasos, the retinue of Dionysus, including dancing maenads and a satyr.

The city remained prosperous into Late Antiquity, when its older Thracian name Beroe came back into use, and it became an episcopal centre. The Regional History Museum preserves part of a Roman street in situ, together with sculpture, inscriptions and the Procession of Dionysus Mosaic.

The modern spa town of Hisarya grew around the same hot mineral springs that attracted people here in antiquity. The area became part of the Roman province of Thracia in the 1st century CE, and by the 2nd century a substantial bathing complex had developed around the springs. An inscription indicates that the area formed part of an imperial domain under Hadrian.

The settlement became a city under Diocletian and took the name Diocletianopolis. At least four public baths are known. The largest covered around 3,000 m² (32,300 sq ft) and contained pools, bathing rooms, and a nymphaeum, or sanctuary of the nymphs.

The most impressive remains are the Late Roman fortifications. Built in the late 3rd and early 4th centuries CE, the walls stretched for more than 2.3 km (1.4 mi) and were reinforced by 44 towers. In places, they still rise more than 10 m (33 ft) high.

Four principal gates gave access to the city. The best known is the southern gate, popularly called Kamilite ("The Camels") because of the distinctive shape of its broken arch. Just outside the walls, a 4th-century CE underground tomb features a vaulted corridor and a burial chamber. It is famous as the only Roman tomb discovered in Bulgaria to preserve an intact floor mosaic.

On the southern bank of the Danube near Svishtov, Novae is the best-preserved example of a Roman fort in Bulgaria. It was one of the main military bases of the Lower Danube. The first camp was established in 45 or 46 CE by Legio VIII Augusta. Legio I Italica arrived in 69 to 70 CE, rebuilt the fortress in stone, and remained stationed there for centuries.

At the centre of the fortress stood the legionary headquarters (principia), built in the late 1st century CE. This large complex contained a monumental entrance, colonnaded courtyard, basilica, administrative rooms and treasuries, and the aedes, where the legion's standards were kept. The soldiers also had access to an extensive bathhouse, which was supplied by an elaborate water system.

By the 5th century CE, Novae was no longer simply a legionary fortress. A civilian town had grown up there, and it became an episcopal centre with a large Christian complex containing a three-aisled basilica, baptistery, smaller basilica and episcopal residence. The settlement survived into the 6th century, when attacks by Avars and Slavs brought its Late Antique occupation to an end.

The modern city of Varna occupies the site of ancient Odessos, a Greek colony founded on the Black Sea coast around 570 BCE. Under Roman rule, it became an important port of Moesia Inferior and flourished particularly during the 2nd and 3rd centuries CE.

Its most impressive Roman monument is the great bath complex, constructed around the end of the 2nd century CE and covering more than 7,000 m² (75,000 sq ft). The baths preserve the main rooms of a Roman bathhouse, including the frigidarium, tepidarium, and caldarium, as well as parts of its hypocaust underfloor heating system. Some walls still stand up to 22 meters high, giving a good sense of the building's original scale.

The Archaeological Museum of Varna is well worth visiting. Alongside its famous prehistoric collection, it houses sculptures, funerary reliefs, inscriptions, glass, coins, and other finds from Roman and Late Antiquity Odessos.

Deultum was a Roman colony founded under Vespasian (69 to 79 CE) for military veterans of Legio VIII Augusta. It is located about 17 km west of Burgas, near the village of Debelt. Unlike older cities in the area, Deultum was a new Roman settlement and operated as a bustling port on a navigable lake connected directly to the Black Sea.

The colony prospered in the 2nd century CE and was fortified under the reign of Antoninus Pius (138 to 161 CE). Excavations have revealed paved streets, public and private buildings, and a bath complex from the 1st century CE, which was later expanded. Archaeologists also unearthed buildings connected to the imperial cult, including statue bases for Roman emperors and a bronze portrait of Septimius Severus.

Deultum remained significant in the late Roman period, with new fortifications built in the 4th century CE. It remained occupied into the Byzantine period, with parts surviving into the Middle Ages as a fortified settlement.

Villa Armira is one of the finest Roman villas discovered in Bulgaria, located near Ivaylovgrad in the Eastern Rhodope Mountains. The villa rustica grew into a luxurious country residence covering about 3,600 m² (38,750 sq ft), with two storeys, more than 20 rooms decorated with Roman mosaic floors, a bath complex and a central courtyard.

The richness of its mosaics and marble decoration is what stands out most at Villa Armira. The courtyard contained a pool ornamented with marble reliefs of dolphins and vegetal motifs, while the floors combined elaborate geometric patterns with figurative scenes. One mosaic shows a bearded man, probably a member of the family who owned the estate, with two children, while another one depicts Medusa.

Villa Armira was occupied and remodelled for several centuries before being destroyed and abandoned during the Gothic War of 376 to 382 CE. Rediscovered in 1964, it has been extensively restored, and its decoration provides a unique insight into elite life in Roman Thrace.