Monitors and Forts

So one might say, if the Grand Human Union is such a peaceful, benevolent place why does the Star Legion have such devastating firepower as embodied in First, Second and Third Raters?
The answer is monitors and forts.
A monitor is a 10 to 30 million cuft warship. They are heavily armored with force fields and armed with massive energy weapons and missile launchers. Unlike battleships they typically do not embark battleriders or troops. They also typically do not have shunting capability. That means that they are limited to accessing subspace using a gate.
That means that monitors are used as both system defense ships and gate defense platforms.
The development of the battleship was a direct result of the deployment of monitors which greatly outclassed the frigates and battleriders that existed at that time.
However dreadnoughts and superdreadnouths were a response to the even larger and more powerful forts developed to defend systems.
A fort is a grav powered installation typically grav anchored on the subspace side of a gate or in orbit near the real space side of a gate. With BG engines typically only within the 4 to 6 g range a fort can move only sufficiently to move to its place of station and make a poor target for ballistic weapons (like unpowered missiles.) Because it does not have to mount a powerful BG engine a fort is not limited to 250 million cuft. As a matter of fact it usually mounts more powerful force field armor than all but the most powerful superdreadnought as well as having a massive structural frame supported by structural integrity fields and physical armor as well as internal armor force fields.
Forts also typically mount massive energy weapons, missile launchers (for use in real space) as well a numerous battlerider squadrons. Forts are often located near each other where they can provide mutual fire support.
Like other combat vessels forts deploy either sensor drone nets or purpose built sensor arrays to enhance their abilities to detect incoming enemies.
Enemies who defeat gate defense forts often find themselves facing another layer of defensive positions on the far side of the gate in real space.