The premise is as follows:
A species uses biological constructs to protect themselves from forces that attempt to exterminate them. These biological constructs have a far faster growth than the species itself, so it settles worlds in advance of the species. Most worlds are terraformed as much as possible by a prior entity, worlds outside of the goldilocks zone are prepared with ease of living inside protected areas.
Species A is a humanoid group consisting out of religious humans who have their gods act through their priests while various supernaturals (werewolves, vampires, mages, various demons/angelics) live among them in relative harmony. So their culture is basically "earth with a twist". The danger of extinction has focused almost all efforts on survival.
The biological constructs, because of the religions of species A, are all humanoids build to combat perfection but without any non-biological implants to enhance them to keep them relatively cheap and use easily attainable materials (these are the guys from my "super soldier" questions). When not in combat their culture is a sort of hive mentality where they'll work in unison on any project to further the safety and population of species A. The cities and towns they build all have a religious hub that is build close to when the humans arrive, and are otherwise completely up to the bio-constructs to build and devise the layout.
Species B is a group of multiple species under thrall of multiple "big" gods who found mankinds persistance to cling to their own gods worthy of extermination.
As the bio-constructs settle the world, they prepare infrastructure for when their owner species arrive. Simultaneously they form a buffer between the hostile forces and the humans. So the bio-constructs build things to last against invasion. I first envisioned cities with city blocks that sport big artillery or cross-fire on each intersection, but this would be far too vulnerable to enemy large-scale attacks. So the question is how would these bio-constructs build up the world to be as hard as possible to exterminate in case of attack, and still make the world as livable as possible for the species once they have the population to start settling it?
- The invading species wants the world to remain habitable after the invasion. No carpet-nuking or throwing asteroids so large the planet needs terraforming again (or more than a year of having to settle down before colonists can live there again).
- Orbital bombardment, similar to airstrikes, are long and expensive campaigns. Also it seems hard to tow enough mass to the planet to destroy its entire surface. Space-based airforces (that go into the atmosphere) would be more expensive to maintain due to their requirement to speed up and rejoin their orbiting carrier.
- There is FTL travel available, but due to it's workings you require increasingly smaller jumps the closer you get to a large mass (like a planet) to not exhaust your energy.
- The bio-constructs can construct food from the raw components, no need to grow it.
- The bio-constructs do have a space-based force, but most of it is used as reinforcements to planets or assaults elsewhere rather than defense. enemies would simply gather more forces than the standing spaceforces anyway and blow them apart, better use them as reinforcements to stop an attack. Due to the scale of the conflict, reinforcements would on average take 2 to 10 years to gather and arrive at the planet, giving the invaders time to mount a ground assault.
- Bio-constructs have access to ground-based anti-orbital equipment. This has it's flaws of course as these require energy to get it into orbit first. On the flip side, the ground forces have far higher energy available than the space ships. FTL-based defenses are also available, but these options can't "fire" its projectiles to a distance closer than 5km of the surface because the energy requirements go through the roof.
- Technology has advanced such that the combination of armor, weapons and countermeasures puts the capability somewhere around the 90's in terms of detection and firepower.
- Early warning time is 6 hours to a week, depending on what direction the attackers are coming from and the position of the planets (FTL needs to avoid traveling close to planets because lf the added energy requirements).
- The bio-constructs are all military capable.