Here's how I think the setting works:
Inside each Tower there is a Ramus. However, looking at the Imperial Tower and what the Garland is undergoing right now, it's probably more precise to say that a Tower is a Ramus' final evolution parasitizing a structure. A Ramus grows through the exercise of their powers on the battlefield. At one point in this process, their "branch" could blossom and the Ramus attains the next level of their power. If the Ramus then continues to survive (which isn't guaranteed - Mirror's blossoming was destroying her body) then at one point their "branch" would turn into a "garland", encompassing their entire head in flowers. The "garland Ramus" would then continue to evolve and at some point they could start encompassing their body in branches, parasitizing a tower structure and forming its core. At this point they would generate a "sovereign", a branch person functioning as their avatar (because the original Ramus body becomes trapped inside the core). They also gain control over a territory that the "sovereign" rules over. Over time, new Rami are born inside the territory and can be used by the Tower to expand their influence, taking over other Towers. Over time too, some of those Rami would blossom, garland, and ultimately turn into a Tower as well, becoming an independent "child" of the original Tower. Most, however, just die on the battlefield.
Towers could also wither away, although it isn't clear whether the "reproduction" process is the cause of this.
What I think the Emperor (or rather, the Imperial Tower of Erin) is trying to do is extend its lifespan by transferring to the new "Garland Tower" being born as it itself withers away.