There are multiple points that have to be established before this starts to make sense. SPOILERS AHEAD
1. The peoples of the tower are not different castes, but different civilizations called to the tower by the Anchorites
The Steam page says "Divided since the dawn of time, the Peoples of the Tower no longer speak to each other. It is said that one day, a Traveler will find the wisdom to break down the walls and restore the Balance". This description is misleading at best, as shown by the facts in the game.
Firstly, the protagonist is not a traveler. They were created by the Anchorites after the Tower was made, so they may have never seen the world outside the Tower.
Secondly, if they were different castes, why would the alchemists need to mine their metals on their level of the tower instead of outsourcing the job to the lower levels or, you know, people on the ground? If the warriors are sending stuff to the Bards as it initially seems to be, why do the Bards need serfs, and why do no warrior assets appear in the Gardens? That's because the levels are not separated just by language but also by economics.
This is why the Anchorite who created the protagonist says: "My people created the tower. Foreign peoples came to the tower". This is shown 2 times:
- The Devotees were dealing with famine or some sort of ecological disaster in their homeland when their God led them to the Tower.
- The Warriors, a society interested in astronomy, saw "The Call" in the night sky and navigated by sea to the Tower.
There's no indication of when the Bards or Alchemists( or to be more accurate their ancestors) came, but this is likely because this knowledge was lost during the schism or evolutionary divergence between Bards and Alchemists.
The poster/image at the beginning of the Anchorite's level shows that this was no accident. They were waiting to welcome new people to the tower. It's likely that the kiosks in the area were not the Anchorites splitting the languages, but instead figuring out how to translate them. However, something happened so that they gave up, instead retreating into Exile. Therefore, by uniting the people of the tower the protagonist is actually fulfilling the Anchorite's original mission. But...why?
2. The "Impure" are real
Notice how the mural of the impure in the Warrior's fortress...looks nothing like the Devotees. They're short, are sometimes on all fours, and have tails. This could be a case of racist stereotyping or demonization, but what if the term "Impure" didn't originally refer to the Devotees? Instead, there actually was a force of these creatures that tried to attack the tower, and the Warriors codified "The Duty" by pushing them back. It was only after this conflict that the Devotees came and were mixed up with actual invaders. This explains why the Warriors are instantly okay with the Devotees coming into the fortress when they prove they can play music: it's proof of their humanity. Another piece of evidence for this is inside of Exile's virtual reality, where the mural is altered so that the Impures are playing music. Since this is a deliberate distortion, it is likely that the opposite is true, and that these monsters have some sort of Creative Sterility.
Perhaps the world is actually After the End, and the tower is an effort to gather and unite what remains of civilization.
In summary, the people of the tower were intentionally lured there, to unify against some mysterious threat. And by uniting the people, the protagonist has finally completed a plan that has been in the making for who knows how long. Maybe a potential sequel could see the protagonist leaving the tower to deal with this threat directly.
