The Imperium class heavy battle cruiser was a type of warship employed by the Romulan Star Empire in 2280.
These ships were made for a more striking design with a massive command boom appearing at the center of the vessel, in between the elegant wings of the ship which gives the ship a near catamaran type of appearance. Strangely though, these ships are rarely encountered and their full capabilities were not known. (TOS video game: Klingon Academy)
The Firehawk (Rihannsu: Llaiir'Dhael) was the heavy cruiser version of the Sparrowhawk light cruiser. This ship is not modular. While it could be constructed with different modules, any given ship carried only a single type of module through its entire service.
A Firehawk-A (FHA) could be refitted with K-modules, producing a Firehawk-K (FHK). This was an extensive refit, however. (TOS video game: Starfleet Command I)
The Novahawk (NHK) was the Heavy command cruiser of the Firehawk series. It only carried K-modules. (TOS video game: Starfleet Command I)
This ship was another Romulan effort to deploy more Type-R torpedoes. A variant of the Firehawk, the Regalhawk (RGK) was less expensive and slightly less capable than the Royalhawk. These ships rarely operated alone, not from doctrine, but because their heavy torpedoes were in high demand for the most important attacks. (TOS video game: Starfleet Command I)
The Royalhawk (RHK) was a limited-production variant of the Novahawk. It was designed to field the massive Type-R plasma torpedo in greater numbers to counter the Gorn Strike cruiser. Like the Novahawk and its more modern cousins, it only carried K-modules. This ship could be considered a Heavy Battle cruiser back in the late 23rd century. (TOS video game: Starfleet Command I)
The ultimate expansion of the Heavy Hawk designs, the Killerhawk (KHK) was essentially an attempt to build a pocket Dreadnought. This design was not considered successful since it placed too many weapons on too small of a hull. The maintenance cost to support this ship was far higher than any other Firehawk design. Only a few were built. (TOS video game: Starfleet Command I)