The Romulan Star Empire was first encountered by Klingon vessels in 1600 IR, an encounter that lead to the first of many battles. Up until 1643, little was known of the Romulans, other than they are treacherous and fight dishonorably from the shadows. That changed when we entered into an alliance and technical exchange program with the Romulans. The alliance-formed as a response to the ever-burgeoning political and economic strength of the Federation-ushered in an era of relative peace between our two Empires.
We learned much from our new allies, as did they, and that has become the source of political strain on the alliance. The Romulans are a proud people who are in the same situation as we. Sorely lacking resources, they must expand or perish. Often, their need to expand has resulted in armed incursions into Klingon space. This has led to many clashes even after the alliance was formed, yet the overriding threat of the Federation has prevented the escalation of hostilities and the alliance remains tenuously intact. Their need to expand, compounded by their devious, secretive, and dishonorable nature, has led to mistrust between our two governments. Agents of Klingon Imperial Intelligence monitoring the Romulans, have recently reported the Romulans are running dangerously low on dilithium. More ominous still, the Romulan Imperial Navy has conducted numerous secret research projects in violation of the technology exchange treaty between both Empires. Imperial Intelligence has yet to discover the exact nature of these projects, but efforts to do so continue.
As with all potential enemies, analysis has been done on Romulan technology and tactics. They have similar technology to our own, including warp drive, impulse engines, transporters, and the like. They perhaps have slightly inferior warp drive capability, but is an intended result of out-dated schematics and data given to them in the technological exchange.
A Particular to the Romulans is plasma torpedo technology. A very powerful seeking weapon with the considerable drawback of degraded performance at medium and long ranges. This is principally due to the torpedo steadily degrading charge after initial launch. The exact cause of this charge degradation is not entirely understood by Klingon science, and obviously not be the Romulans, otherwise this limitation would have been ameliorated by this time.
This fundamental limitation in their main weapons system has actually guided the development of Romulan starships and the tactics they employ. Because these torpedoes take a long time to arm and they are most effective at close range, the Romulans use their cloaking devices for "hit-and-run" tactics. They approach as close to an enemy as possible under cloak, decloak, then launch a volley of all their weapons, cloak again to move away and recharge. This tactic has lead to the Romulans forsaking strong shields in favor of armored hull plating since the cloaking device negates the use of shields, as well as to a high degree of maneuverability to bring their weapons to bear quickly after decloaking.
Because of the design limitations of their weapons and warships, Romulans by necessity fight without honor, they constantly hide their face from their enemies. It is generally accepted amongst scholars that this fundamental flaw in their warior's code, combined with their rapidly increasing isolationism and lack of resources, is turning this once honorable and worthy race into a devious, deceitful, and disgraceful shadow of their former selves. There are those within the Empire who believe their decadence is not merely a sign of their cultural decline, but a social contagion. This belief has fostered a growing movement to dissolve our alliance with them lest their sickness spread across the border and start to decay the moral foundations of our own Empire.