The site at Llanedwen, Anglesey, revealed "an unusual amount of high status material, suggesting a Roman site with links to the military".
The Gwynedd Archaeological Trust says road works and a honeycomb of buildings were found, but no defensive ditches.
Pottery and coins found were also discovered at the site.
The works at Trefarthen was funded by historic monuments agency Cadw.
Analysis of the pottery shows the site dates from the end of the 1st Century, through to the middle of the 4th Century.
The site is located opposite Caernarfon, which is home to Segontium, a major part of the Roman military presence in Wales.
Dave Hopewell, of Gwynedd Archaeological Trust, said: "It changes our view of how we see Roman Wales really, and Roman Britain to a certain extent.
"It has always been assumed that we were in a militarised zone here and there was no Roman or Romanised civilian settlement at all."