Soma's Castle in "Dragon Quest III: HD-2D Remastered Edition" is a unique area in the game. If you want to go to this place, you can first go to Tantegel. When you get there, you will Soma's castle was seen rising in the distance, shrouded in purple mist.
How to get to Soma’s Castle in Dragon Quest 3 Remake
Reaching Soma's castle is one of the main puzzles of Alfgard. Once you reach Tantgarh, you'll see Soma's castle rising in the distance, shrouded in purple haze. Although the island is right in front of you, on the opposite side of the coast, it is surrounded by mountains and hills and cannot be reached by sea.
Therefore, you need to get Rainbow Didi. The prerequisite for obtaining Rainbow Drops is to collect three items: Tantegel's Sun Stone, the Rain Staff from the Soul Temple, and the Holy Amulet from Ruby's Tower. Once you have collected these three items, you can take them to the temple in the southeast, where you will receive the Rainbow Drop.
Next, you only need to follow the guidance of the priest and go to the peninsula north of Cantlin, where the rainbow drops will form a bridge, leading you to Soma's castle.