Concrete was one of the main innovations of the Romans. Also, the arch, vault and dome.
You will have seen how poor people lived in crowded insulae in the cities (although not in the countryside). Villas offered space for the wealthier members of society.
The local aristocracy who may have seen the benefits of collaboration with the Romans or who wished to buy into the culture of the occupiers.
They were similar in design and methods of construction, although could vary in terms of complexity, size and decoration.