This article is from the Final-Fantasy and other Square Soft Games FAQ, by email@example.com (Nick Zitzmann) with numerous contributions by others.
First off, most of the doors are locked. Find the key to unlock them -
it should be somewhere near the entrance to the castle.
After a while of exploring the castle, you should find three rooms - one
with a horizontal line of orbs, one with a vertical line of orbs, and
another one filled with orbs. Back in the town, there was a person who
told you to "remember where the two lines meet." Keep this in mind, and
use the clues in the first two rooms to find where the lines intersect.
Find where the "lines" intersect in the third room. If you guessed
correctly, Sei-Ryu will attack the party. If not, you'll be attacked by
an AtomCrab instead.
Give up? Read on...
Go to the third room. Select the orb which is two orbs away from the top
and two orbs away from the right. That orb hides Sei-Ryu.