This article is from the Final-Fantasy and other Square Soft Games FAQ, by email@example.com (Nick Zitzmann) with numerous contributions by others.
It's on Floor 21 of the Tower. Find the exit from the tower on this
level, and you'll find yourself in a world full of flowers and
mountains. Somewhere in this world is a straw hut with an old man lying
in bed. Talk to him to get the Excalibur.
The Excalibur, when used, will attack a whole group of enemies at once.