The earliest surviving structures at Umm el Jimal date from the Nabataean period — the 1st century CE or possibly earlier — and include a triumphal arch and the foundations of several buildings that represent the site's origins as a Nabataean waystation. The Nabataean arch, though partially ruined, is one of the more photogenic elements of the site and marks the approximate centre of the original settlement.
Nabataean inscriptions found at the site provide evidence of the trading community that first established itself here, drawn by the site's position on routes connecting the Hejaz with the agricultural regions of the Hauran. The transition from Nabataean trading post to Roman military town happened after 106 CE, when the Roman emperor Trajan annexed the Nabataean kingdom and reorganised the region as the province of Arabia — a shift visible in the changing architectural styles and building programmes recorded in the ruins.