Gwell Wartime Doctrine

The Gwell are a species that live inside other huge creatures, specifically the Leviathan Troznux.

Gwell can hide within many spots within the organ systems of a leviathan. Other species might be present as well, but the gwell can hide in a huge variety of imperfections within the exterior areas of the leviathan's organs, as well as hiding behind objects like muscles, tendons, small bones and more. Additionally, the very dark nature of the internal systems means that the gwell have no eyes and will not be able to see their enemy. Because of this, it is entirely possible that they establish a sort of guerilla doctrine. There would be few large vehicles for warfare so they might rely more on tamed creatures, and logistics is made quite easy because they can exploit the open vascular system of their leviathan to travel long distances quite easily and quickly.

Depending on which organ they inhabit, the battlefield conditions might be changing constantly. For example, the leviathan's hearts will always beat constantly which would completely redefine any tactical moves made there. Any soldiers deployed on it would need to stick to it despite the incredible forces that would act on them, but so would the enemy. Some generals have been known to exploit this.

Gwell battles probably happen at much shorter range to the point where a melee doctrine might even work. 

The brain always has a much lower population, if it has a population at all because of a large number of barriers which disallow entry. This has become a thing because of other creatures historically managing to enter its brain and cause great damage to its host, so there was an evolutionary push to seal this entrance off. 

A leviathan's intestine has a very complex geometry which would be incredibly difficult to plan battles on. Any soldiers sent to such organs would need to be equipped with devices that allow them to stick onto the leviathan's organ more effectively, and large scale guerilla warfare becomes quite easy. Ranged weapons might become more widely used to attack enemies across the airgaps in the intestine. The fairly loud nature of this organ also would help to mask any sounds made in the battle. The interior of the intestine might also hold a population, but the doctrine within it might be to attempt to make the leviathan digest the enemy forces. This might be done by making the enemy lose their grip or balance and making them fall into the various materials being digested by the leviathan. Sometimes, particularly large clusters of this material might pass through so the goals might change to preventing the enemy forces from moving. 

Most other organs are relatively similar, often taking a large roughly spherical appearance which is large enough to appear flat to any gwell standing on them. Muscles might be too dangerous to fight on because, in the case that it tenses, it might squash all forces between the muscle fibers and lead to total destruction.

Battles might be fought on the outside of a leviathan too, with the creature's gravity keeping forces bound to its skin. Light may be available, so there might be a subspecies of gwell that can sense the light. There are still a lot of imperfections in the leviathan's skin on its exterior, but its likely that these cultures make much more use of long range weaponry. They have less metal available because they do not have ready access to blood, but they can easily receive energy from sunlight by creating farms that photosynthesise, or exploit the leviathan's own ability to photosyntheseise.