“Marth did as Gotoh instructed and chose soldiers for his journey into the unseen. What lay within the tower in this alternate world to test him? Marth's heart wavered. But with neither the divine blade Falchion nor the divine dragon girl to aid him, the tower's master was his last hope of defeating Medeus. Gotoh had told Marth to find a coffin. So while most of his companions remained in the material world to battle Doluna, Marth and his chosen few scoured the tower to search of a mysterious savior.”
The Alterspire is Chapter 24x of Fire Emblem: Shadow Dragon.
This chapter is the only chapter to contain enemy Berserkers,Swordmasters, and Warriors, (except for the chapter The Lair of Fire, where Ymir is an enemy before you recruit him) although you can fight units of these classes in the arena.
There is a way to enter this chapter without losing Falchion. First, the player must have Tiki hold onto Falchion. Second, they shall allow her to be killed in or before Chapter 24. At the end of the chapter, Tiki is dead and Falchion is gone, so Gotoh sends the group to 24x... But, since Tiki died while holding it, the good Falchion will be in the inventory. Third, then, the player must use the Aum staff to revive Tiki. Following this, it is possible to enter the Endgame with two Falchions and two Divine Dragons.