Adjective clause is a type of dependent clause that consists of a subject and a verb. As It functions as an adjective, it modifies a noun or a pronoun. This type of clause begins with certain relative pronoun such as that, which( to refer to things) and who and that (to refer to people).
In this example, the clause “who were fond of their comforts” is the adjective clause which functions as an adjective as it modifies the noun “Romans”.