This is the 15123rd most frequent Polish word.
rzymskiej
"Rzymskiej" means "Roman" (adjective, feminine singular or plural genitive/dative/locative form).
In this sentence, 'rzymskiej' is used in the locative case, indicating the era as Roman.
Inspiracje pochodzą z rzymskiej epoki.
The inspirations come from the Roman era.
The word 'rzymskiej' is an adjective in the locative case, describing the civilization which is Roman.
Księga opowiada o historii rzymskiej cywilizacji.
The book talks about the history of Roman civilization.
Here, 'rzymskiej' is in the genitive case, modifying 'architektury' to mean architecture of Rome.