theromanempire.org
The definite article does real work. It makes the name read as the subject itself rather than as a description, which is the right register for an authoritative resource. It sounds like the reference rather than a site about the reference.
The Roman Empire is among the most studied and most popular subjects in human history: school curricula, university programmes, documentaries, podcasts, games, and a readership that hasn’t diminished in five hundred years of printing.
On the .org it suits an educational resource, a museum or archaeological project, a learned society, a digital archive, or an academic publisher. Classical studies live almost entirely on that extension.
The content opportunity is real, since what currently serves the subject online is a mix of ageing university pages and thin material.
Three words, exact subject, no hyphen.
The name is listed. Enquiries welcome.