The Celts had a huge network of roads. That is one of the things commonly attributed to the Romans. Much of what is attributed is no doubt exaggerated because they wrote much of the history and discounted the accomplishments of barbarians.
Centralized government.
Roman style warfare.
Roman Catholic Church and Christianity.
The Romans had aqueducts and other modern things like indoor plumbing but they have been reinvented so they aren't really due to the Romans. They invented cement and concrete.
