Following the establishment of the Roman Republic in the 5th cent. B.C., Roman architects began to absorb and synthesize influences from both the Etruscans and the Greeks, adapting earlier building types to their specialized urban needs. A characteristic feature of Roman design was the combined use of arcuated and trabeated construction (employing arches and constructed with post and lintel). Although at first tentatively employed in the spaces between the classical columns, the arch eventually came to be the chief structural element. Flanking columns, usually engaged and superimposed (partly embedded into a wall and laid over it), served merely as buttresses or for decoration.
The cut-stone construction of the Greeks was largely replaced after the invention of concrete in the 2d cent. B.C. This enabled architects to cover vast interior spaces with vaults of increasing complexity and without interior supports. These included the barrel vault, the cross or groined vault, and the dome and semidome. Vault buttresses, instead of forming exterior projections, became an integral part of the interior support system. Although unfired brick was employed in all periods, under the empire baked bricks became popular as a facing for concrete walls. From early times stucco was used as a finish for important buildings. For the more luxurious finishing of exterior and interior walls, sheathings of alabaster, porphyry, or marble were used.
Of the Republican period (c.500–27 B.C.), the great aqueducts outside the city of Rome are the most impressive remains, but monumental architectural ruins also have been excavated at the city-state of Gabii. Dated to c.350–250 B.C., the formative period of Roman architecture, the Gabii remains consist of a large complex with stone walls and terraces connected by an impressive staircase and surrounded by rooms as well as other structures. Ambitious and grand in scale, they appear to contradict the small and modest reputation ascribed to buildings of this period; the influence Gabii may have had on the growth of Rome is unclear.