Cardiff began as a Roman fort. The Romans invaded Wales about 50 AD and about 55 AD they built a fort on the site of Cardiff. In the late 1st century the fort was reduced in size as Wales was now at peace.
However in the mid-3rd century the fort was rebuilt and strengthened to defend South Wales against Irish raiders. Yet in the fourth century Roman civilisation declined. Towards the end of the century the Romans abandoned the fort at Cardiff. www.localhistories.org/Cardiff.html