It's best you follow this hint if you've played the NES version or had at least one playthrough for FF1 on Dawn of Souls since known landmarks are used.

After defeating the Lich in the Earth Cave, the party can speak with the Circle of Sages in the town of Crescent Lake for a canoe; with it, the party can now cross river terrain.

Take the ship out of the Aldimeer (Aldi Sea) then north to the island where the Castle of Ordeal is. Enter the river terrain and then get on land to access the castle. Note that the enemies here are probably more powerful than low and mid-level parties can withstand, so come prepared.