Short answers, pulled from the story.
The Temple of Portunus in Rome dates from the 2nd century BC. This structure reveals how Roman builders first adopted Etruscan models before embracing Greek styles around 200 BC during the late Republic.
Roman temples typically sat on platforms raised higher than their Greek counterparts, sometimes featuring ten or twelve steps instead of three. Unlike Greek temples which treated all sides equally, Roman versions emphasized only the front facade while leaving the rear walls largely undecorated.
The Pantheon in Rome represents the most famous example of a circular design, featuring a magnificent concrete roof behind a conventional portico front. This building remains unique among surviving Roman temples due to its massive scale and engineering complexity.
Vitruvius, the only significant Roman writer on architecture to survive, described idealized proportions that did not always match actual practice. His writings covered Tuscan orders but failed to recognize the Composite order later formalized by Renaissance observers.
Emperor Honorius issued a decree in 415 AD that delayed the appropriation of Roman temples for Christian use. Most surviving Roman temples underwent conversion into Christian churches or sometimes mosques centuries after their original construction following this event.