When the Vulcans adopted pure logic as a philosophy, those Vulcans who could not accept the new worldview left in huge ships. Some of these reached Remus (very much like Vulcan), Romulus (a class-M planet that became the capital of the Romulan Empire), and other planets. Over a period of a thousand years, these ex-Vulcans lost and then regained space technology.
The Romulan Empire is run by an Emperor, with the support of the Great Houses (the 21 powerful families who control most of the Romulan economy). This creates a great deal of inefficiencies and waste as each Great House is more interested in its own advancement than in progress for the entire empire. The refusal of the Great Houses to work together (along with a few Gorn commando raids) delayed the Romulan development of warp power by decades.
The Romulan fleet is based around three series of ship; their original Eagle series, the Kestrel series obtained from the Klingons, and the new Hawk series.