The Osohe Castle is an area near Tazmily Village that holds two secret items. The Drawbridge is locked until later in the game, when Duster unlocks it.
Osohe Castle is a house to many Ghosts. Most of them are really nice, and will sell things, talk to you, etc., but some of them will fight you. This is a way to get Rotten Eclairs, which is currency while you are there. While traveling throughout the Osohe, you will also find a girl's pendant. After walking a little bit more, you'll find a sacred treasure. If you go even further in, you'll see the Hummingbird Egg, which is an item that holds the memory of everything in the past. You'll find Kumatora, the princess of the abandoned Osohe Castle here as well.
At the end of the game, during the Armageddon, Osohe Castle is destroyed, like the rest of the world. The castle sinks into the ground.