You fight enemies on each tower floor and meet different foes and bosses across its zones. The main tower has 5 zones, and clearing floor 50 opens the way into the abyss.
In "Tower", open "Floor" under the formation and target floor controls to choose a destination. Read "Scouting Intel" before battle to inspect the enemies and risks on that floor.
Return to a cleared floor to train mercenaries or gather equipment if your next floor defeats you. On entering a new zone, check the enemies before committing the party that served you in the previous one.