Across the galaxy, defensive installations are used to protect and secure important locations from space based threats. They generally have a better cost to effectiveness ratio compared to warships, though lacking the mobility. There are 4 main types of defensive installations in common use across the galaxy, though some groups use specific types more prolifically than others. They are air bases, weapons platforms, ground to orbit weaponry, and battlestations.
AIR BASES:
Air bases are one of the most common types of defensive installations in Alliance space, though it is relatively rare in the rest of the galaxy for direct defense, usually being used for policing. Air bases serve as the storage and deployment facilities for large numbers of fighters and small ships like corvettes. In less bellicose regions, they reside on a planet's surface or in thin-skinned space stations for cost efficiency. More defensible air bases are mostly deep under a planet's surface or inside a large, usually metallic asteroid, with only some components on the surface when necessary such as launch openings and sensors. They are also commonly in hidden locations protecting themselves with secrecy instead of barriers.
WEAPONS PLATFORMS:
Weapons platforms are, usually unmanned, space stations which are entirely centered around their weaponry, with limited considerations for other systems besides maybe maneuverability. There are 3 types of weapon systems that are usually deployed on weapons platforms: mass accelerators, lasers, and missiles. Generally, primarily missile and laser based platforms are on the smaller side, due to lack of need for missiles and heat dissipation concerns for lasers. Mass accelerators, however, tend to have large platforms due to the increased range and effectiveness of larger mass accelerators. Most mass accelerator platforms use cruiser grade mass accelerators as a balance of effectiveness and power though dreadnought and frigate grade platforms exist. Platforms are generally more maneuverable than most warships due to not having considerations for long range or FTL. Many weapons platforms are not even powered by on board reactors, instead being beamed power from another location, usually a surface based reactor or even a solar collection array.
While some weapons platforms are manned, many are unmanned, either controlled by VI or, more often, remotely operated. Unmanned platforms are called weapons satellites.
So called super lasers created by using orbital mirrors to focus solar energy into deadly beams of light are occasionally seen, but their delicacy means they are not particularly effective, usually only seen as impromptu weapons repurposed from other roles.
GROUND TO ORBIT WEAPONRY:
The first type of anti-space defensive installation created by most races, ground to orbit weaponry is the most varied type of defensive installation, and the one that most stretches the term "installation", the term being more a catch all for anything used to attack ships in orbit from the ground, even those that are mobile.
Ground to orbit weaponry generally consists of either mass accelerators or missiles. Any mass accelerator capable of doing significant damage is usually mounted in stationary batteries, with only the occasional deployment in more unique ways like on massive submarines or ocean going ships. Stationary batteries are tempting targets for counter battery fire from orbiting ships, leading to most such installations being heavily protected. Kinetic barriers are a common protection, but they can be battered down quite easily unless upscaled to rather ridiculous size. Due to this, most are protected by thick physical barriers, sometimes actual armor, but the high cost of any thickness of armor actually capable of defending against mass accelerator rounds means that armor made of rocks, concrete, and simple metal armor with a final layer ship grade armor is more common. Underwater mass accelerators, using mass effect fields to open a commodore to the surface, have proven very effective, providing both great protection, and cooling for the weapon.
Missiles have far more variety in transportation. Traditional missile silos, both surface based and underwater, are still in use, providing a relatively cheap but effective system. They are rather easily destroyed once discovered, but if it has already launched all its missiles, then it is a small cost. Missile carrying ground vehicles are also in use, though their limited carrying capacity leads to them being less common. Submarine varied missiles are the most popular, particularly with the nations of earth. The ability to hide beneath the waves, moving around after each attack, gives them great survivability like the ground carriers while also retaining a large missile capacity.
BATTLESTATIONS:
Battlestations sit in the odd position of having most of the characteristics of warships, sometimes even having FTL capabilities, but not considered such. The divide between battlestations and warships is paper thin, and very unclear in some places. Take Arcturus Station as an example, it is FTL capable and has all the capabilities of a warship, yet is considered a battlestation. In reality, the divide between a battlestation and a warship is mostly arbitrary, though there are common design differences found in most, though not all, battlestations.
Battlestations usually have far more armor than any equal sized warship. This Is due to the main advantage of battlestations over warships, the lack of need for range, also usually meaning they have lesser maneuverability, so they can't dodge rounds as effectively. Due to this, they need to be able to take more punishment than a warship, and barriers both can't be upgraded as much on a similar sized hull, and cause a lot of heat buildup. A battlestation's armor can be up to dozens of meters thick, with some designs including even more armor. For freestanding battlestations, this is usually made from thick layers of dense materials, mass not being much of a concern if it doesn't need to move. Battlestations can also be built into asteroids, usually using the bulk of the asteroid as protection, though these battlestations can blur the line between battlestation and ground to orbit weapons depending on the size of the asteroid and the ratio of asteroid to man made materials.
Larger battlestations have problems with heat dissipation, their surface area and available heat dissipation locations being outpaced by the volume and heat buildup. The main way this is dealt with is through a heat sink, which can be extremely large for battlestations. Some battlestations simply have a heat sink that, once the battle is over, cooled back down and its stored heat radiated out into space. Some, however, can dump their heat sinks and replace them when necessary. This is the standard for citadel race battlestations but is unknown on human battlestations, though they can be rather easily modified for it as they keep their heat sinks near the outside of their hulls as additional armor.
Battlestations have the same weapon systems as any warship with the exception of spinal mass accelerators, as they lack the turning ability to make them effective. Instead, they have extremely large turreted mass accelerators, usually a number of spinal mass accelerators mounted together.
Battlestation commonly have additional roles such as being an administrative facility or Relay gateway station. These battlestations are usually less powerful than an equal mass pure battlestation, but are still effective in their roles.
The Alliance uses a classifications system for battlestations based on the one used by the United North American States Space Force since the 2140s. The type is decided based on its mass. The classifications used are Fort, Castle, Keep, Fortress, and Citadel, though the Bastion was added and put in between Fortress and Citadel, bumping up the size of Citadel battlestations for rather obvious reasons.
Forts are battlestations of under 100 thousand tons. These are commonly used in safer areas for security purposes, able to easily destroy the sloops and corvettes commonly seen in the hands of pirates, though outmatched by larger vessels. The Alliance operates several Forts as gateway stations, though the expansion of Alliance Space has led to construction being far outstriped by demand. The Alliance Scoria class gateway stations serve as a good example of a small Fort at 30 thousand tons, designed to deal with civilian traffic through the Mass Relays.
Castles are battlestations of under a million tons, and are rather ubiquitous among minor powers as an economical solution to defense. Many minor colonies have a defense force anchored by a few Castles and weapons platforms supported by small warships as an economical defense against pirates, the warships hunting down the small raiders while the Castles can challenge anything most pirate bands can throw at them. A good example, the Branoric class is an older, larger Castle of Hegemony design. At 900 thousand tons, the Branoric serves as the Hegemony Navy's main anchorages in its early days. Though aging and small for the Hegemony's current needs, each Branoric has taken on a form of their own, being expanded and modernized to completely different specifications, turning each into a unique testament to the Hegemony's presence across the Verge.
Keeps are battlestations of under 10 million tons that can match and even exceed dreadnoughts in power. Keeps are usually only found in important systems and guarding vital Mass Relays. The 8 Vigilant class Keeps are the defenders of the turian home system's Mass Relay, each of the 6 million ton stations having been built during the Krogan Rebellion and been in operation since.
Fortresses are battlestations of under 100 million tons. These rare stations can repel entire fleets on their own, often serving as the centerpieces for entire campaigns and are the prides or headquarters of many navies. Valadari Station is an ancient asari Fortress built during the Rachni Wars to defend Thessia from rachni attack. The 40 million ton station now serves as the headquarters of the Republics Defense Coalition, the largest military packet in Asari Space, which coordinates anti-piracy actions and other military actions, though much of the station has been taken over by the Valadari Museum of Military History, one of the largest collections of military artifacts in the galaxy.
Bastions, originally called Citadels, are battlestations under a billion tons. Rarely does a century include the construction of even one of these stations, the last time multiple of these stations were built rapidly was the Guardian Program, an attempt to defend Council Space from potential attack by the Geth. However, the turmoil of the time led to the program facing challenges. Of the planned 8 300 million ton battlestations, only 5 even began construction. Only 3 were completed to military specifications, one stripped for parts in order to complete the others while another was sold to a civilian company, a fate one of the completed stations would join in after a century of operations. Now only 2 of the stations remain in operation, and declassified spying operations into Geth Space show similar stations opposing them beyond the Relays. The Alliance's 228 million ton Arcturus Station is planned to serve as both the main headquarter of the Alliance, and its last defender before Sol.
Citadels are a theoretical battlestation designation beyond a billion tons. No Citadels have ever been constructed, the exorbitant cost making it infeasible. Countless designs have been proposed over the millennia, the most famous being the Invincible, a 3 billion ton battlestation proposed by a Hierarchy Admiral during the Krogan Rebellions in order to end the conflict. Some records seem to suggest that the protheans constructed battlestations of this size, but this has been never proven and no battlestation of this size has ever been found.
Battlestations are usually not alone, as that would be a massive waste of resources. Most battlestations serve as part of an integrated defense network with weapons platforms and warships, the former providing increased firepower while the latter allows for more flexibility.
