This article is from the Final-Fantasy and other Square Soft Games FAQ, by email@example.com (Nick Zitzmann) with numerous contributions by others.
There is an invisible man in Coneria Castle. Search for him on the
ground level of the castle, in a room located around the northwestern
edge. You'll know when you've found him when you find a place which
looks like flat ground, but you can't walk through it.
Talk to the man. He will say one of two messages, depending on if you've
rescued Princess Sara or not.