From around 10 BCE the territory of Slovenia was subsumed into the Roman Empire; Emona (Ljubljana) was the first Roman town to be established, followed by Poetovio (Ptuj) and Celeia (Celje). Some of Slovenia's best-preserved Roman remains may be found in and around these towns. Other significant Roman archaeological sites in Slovenia include Sermin near Koper and Kučar near Podzemelj, where excavations have chronicled settlements stretching back to prehistoric times.

In the 5th century the Roman Empire began to disintegrate in the face of invasions by the Huns and Germanic tribes, and by the 6th century migrating Slavs from the Carpathian basin had begun to dominate the territory of present-day Slovenia.

Successfully resisting the Turkic Avars, they eventually formed the first united Slavic political entity, the duchy of Carantania, based at the Krn Castle, north of today's Klagenfurt (Austria).

One of the largest and most important post-Roman sites in the eastern Alpine region is Tinje, above Loka pri Žusmu, where excavations have revealed seven partially-destroyed wooden houses, parts of which may be attributed to the period from the 7th-9th centuries. Recent scholarship has shown the site to be of crucial importance for understanding the ethnogenesis of the Slovenes - the contact between the Romanised indigenous population and the immigrants Slavs is evident in the Roman wall with two towers intended to enclose the immigrant Slavs. Detailed documentation on the site can be found in the Opera Instituti Archaeologici Sloveniae series issued by the Scientific Research Centre (ZRC SAZU), Slovene Academy of Science and Arts).

Solitary churches on hilltops are a characteristic feature of Slovenia, which experts link to pre-Christian religious groups and the high shelters of the earliest inhabitants. Some of the churches, which are today mostly in the Baroque architectural style, can be traced back to early pagan times. The most illustrious example is the Church of the Assumption on the island in the lake of Bled in Gorenjska sub alpine region.

The late 10th century saw the emergence of Romanesque architecture, with its thick walls, round arches, sturdy piers, groin vaults, large towers and decorative arcading. Significant Romanesque monuments in Slovenia include Podsreda Castle, the Church of the Assumption of Mary in Koper, the Parish Church of St Martin at Laško, the Church of St George in Ptuj, Mali grad Chapel in Kamnik and the Basilica of the Cistercian Monastery in Stična.

Mali grad ('Little Castle') once comprised a wider fortification but this was torn down in the 16th century, leaving only a two-storey Romanesque chapel with a crypt constructed from the 11th to the 15th centuries. The basilica at Stična Monastery (mid 12th century) contains a nave and aisles, transept and five apses and is the only surviving example of the archaic Cluny II type in Europe.

During this period urban churches generally had simple multi-nave plans; an exception was the Church of St George in Ptuj, which acquired a west gallery in the mid-13th century.

Also characteristic of this period were decorated stone portals and stone window panes with banded ornamentation - undoubtedly the work of itinerant masons from Lombardy - which may be seen at several sites, notably at Mali grad Chapel, in surviving fragments of the 12th-century Romanesque palace at Castle Cmurek, and in the earliest Romanesque section of the Chapel of St Martin (formerly St Venceslav) at Domanjševci in the Pomurje Region.

From the early 13th century, Romanesque architecture gave way to Gothic, with its characteristic pointed arches, ribbed vaults and flying buttresses. The monastic orders proved to be the principal catalyst in the spread of Gothic architecture in Slovenia. Important examples include the Benedictine Monastery at Gornji grad, the Cistercian Monastery at Kostanjevica, the Carthusian Monasteries at Žiče and Špitalič and the Monastery of the Order of Teutonic Knights at Križanke (Ljubljana). Urban churches built by mendicant monastic orders typically featured long, narrow presbyteries, eg at Kostanjevica and Gornji Grad.

Later stylistic influences came from Bohemia (the workshop of Peter Parler) via Vienna and Hungary, Parler’s influence is evident in the cut stone details in the old presbytery of Hajdina near Ptuj. The Podravje region (Štajerska) was geographically close to centres of Gothic art in Central Europe and the influences of Gothicism can be seen in St Mary’s Chapel at Celje Abbey Church. In areas closer to the Mediterranean, simple Italian vaulting systems were common, whereas at Holy Trinity in Hrastovlje (14th and 15th centuries) in north Istria the Romanesque tradition remained dominant. At the same time parish churches in Kranj, Škofja Loka, Radovlica, Šentrupert and Cerknica were rebuilt and modernised – their original presbyteries were preserved but their naves were heightened with side aisles into a hall church space with gothic vaults. Smaller provincial churches had exposed roof structures or timber ceilings.

During this period coastal towns began to grow under the influence of Venice during this period and many castles were fortified against Ottoman attacks.