This is a direct reference to a character in the game Dragon Warrior.
In the first castle, there is what appears to be an empty space right above Queen Jane's room and below the person going right and left. Check the lower right corner of this area to find out that there is an invisible person here. Talk to him and he will tell you something before and after you save Princess Sarah.
In Elfland, just beside the White Mage shop, there are three tombstones. Two of them are ordinary tombstones, but the one on the left is different. It is the tomb of Erdrick. Erdrick is a character from Dragon Warrior, another very popular game series produced by Enix. In the Japanese version of FF1 (and in PSX's Final Fantasy Origins), the tomb says ''Here lies Link'' instead. Link, of course, is the hero of the Legend of Zelda series.
While on the world map, press B and Select at the same time, and you will see the world map.