
1. How much better are the defensive platforms? For example, if we can assume that a purely defensive installation costs about, say, a quarter of what a similarly powerful ship costs, then it may be economically feasible to divert enough resources from shipbuilding into constructing citadels and setting up asteroid bases (which also need to be towed into position). Otherwise a fleet is going to be the better option because it can be massed to intercept the enemy, especially considering farsight.

2. A (semi)-stationary defense is only a good investment if it is placed around a asset which the enemy wants to capture or if it is set up in some kind of a bottleneck. Bottlenecks are quite impossible in space, thus it only leaves valuable assets, usually planets. Are the combatants more inclined to capture civilians or maybe just the planets? After Seren both powers are more inclined to exterminate the other side then to capture civilian populations, thus ground wars are going to be quite different from space invasions. Why defend against something what the enemy will not attempt?
3. Umiak consider the Loroi civilian populations to be non-assimilatable and captured Loroi, like on Seren, were simply eradicated . It can be assumed that the Loroi will do the same to the Umiak. It appears to be that at this point it is easier to eradicate enemy civilian populations and then re-settle the planet, because the enemy civilians can not be pacified in a reasonable timescale. Thus it is not very likely that any side will consider wasting too much resources on extensive ground forces, because it is easier to kill everyone from space and then re-settle the planet. If any power can destroy the other side's fleets, then it can besiege its planets for as long as they see it fit. If someone surrenders, especially allied races, then they will be spared, others will be killed. That is one possibility why at the end of any era there were massive orbital bombardments.

4. How will the ground war be then? If any side can win the space battle in a certain system, then it can continue the assault in the next ones. Meanwhile any planets behind the new front lines can be blockaded and besieged. Since they can not contribute anything to the losing empire anymore they are essentially lost until the system can be recaptured. The winning empire can simply decide to leave them and all their defensive installation be until the war is won and then demand a surrender. Or it can take its time to bombard it from space with whatever it takes. Even if the planet is heavily defended, something will still come through and decimate the defenders and the defenses. If the goal is to eradicate the stubborn population anyway, the options are practically unlimited, save for a new ice age or a complete glassing of the surface. The only exception is if the planet is too valuable to pound, for example if it contains Soia artifacts or extensive industry. There a full-scale invasion might be worth the effort.
5. How to protect oneself then? Any defensive installations need to cross the most likely plans of the enemy.
- Only those planets the enemy wants to capture intact need to be heavily defended. However since those are most likely at the core of the empire, the war is usually lost if the enemy gets this far. It can however buy some time to fall back, regroup and then to start a counterattack.
- The defenses of any planet the enemy does not want to capture intact do not need to be that extensive. Instead they should only stall the enemy and give some time to evacuate if the planet cannot be defended properly. Any planet one side has no intentions to capture intact and the other side having not enough power to protect will be simply added to the Charred Steppes. This is very likely the other reason why many planets were heavily bombarded in the past. After the war the winning empire hopes to gain so many new resources and planets, that a few "lost" ones do not matter.
- Even big citadels can be destroyed by a determined assault. It may appear as a viable tactic to let the enemy bleed out assaulting a fortified position, but the history of this war shows that even the Umiak are not that stupid. Since there are no bottlenecks, any fortress can be flanked. The point is, can the enemy be tricked into losing more resources to attack such a fortress then the fortress was worth?
- Fortified positions can assist defending fleets in a decisive battle and turn the battle in one's favor, but since it was already attempted (and not very successfully), how are the chances that it can be repeated and are those chances good enough to consider building the fortifications? Will the enemy really accept the challenge on your terms?
- Fortifications can free up some of the fleet from guarding duty, thus giving one more offensive power. Combined with the farseeing blocker, this might be a viable tactic for the Umiak to defend outposts and the like. The Loroi with their farseeing can however use their defensive fleets with pinpoint accuracy and do not need many fixed defenses.
P.S.: I intentionally omitted any specific technologies and their practical appliances. Is Outsider really such a hard sci-fi? And even if it is, then we should give the author the freedom to implement them as he sees fit. Which of course won't stop us from nitpicking later on

P.P.S.: Embrace the insanity which stems from my wall 'o' text of DOOM! Ahem, I mean sorry for the long post...