Consult these maps for part 7:

You are now in control of Samanosuke. Walk straight, pick up a Medicine, and save your game.

• Item: Medicine

Once everything's done, keep moving forward. There is another Samanosuke waiting for your in the next area, but don't worry you should be fine killing yourself. 

Boss: Stylado (Samanosuke's clone)
He is not really tough, simply use all the magic on him and he will be dead in no time.

After that, advance to the next section until you see 2 chests.

• Key Item: Evil Plate (this is the key to unlock the strange door at the Keep).
• Key Item: Wood Ladder

Just turn a little bit left, you'll reach a dead-end. Now use the wooden ladder you just picked up from one of the chest here, then climb up.
NOTE: if you accidentally cut the robe early in game in the room above. you can still pick up the Power Jewel here.

Go to the green sealed doors, use the Evil Plate to open it. Watch a really cool cutscene (You know in the early 20s this was super cool). As Kaede said, you have to find something to break the seal to get into the Demon World.

Go back to the Sentry Room; side room #2 of Keep's main hall. The moth man will automatically ask you to the Dark Realm but I recommend you choose "No" and save your game first, then re-enter. The moth man will ask you again so select "Yes' this time. to go into the Dark Realm. It's an optional dungeon but the reward is worth the run though. 
Note: you can go to the Dark Realm as many times as you want.

Dark Realm
There are 20 levels with the following reward (they may be different if you go to the Dark Realm by asking another moth man; the other one is in a well):

• Fluorite
• Medicine
• Burst Bullet/Soul Absorber
• Fluorite
• Herb
• Talisman
• Herb
• Fluorite
• Herb
• Bishamon Ocarina (level 20)

These are key items that can only be obtained once:

• Key Item: Fluorite (x3)
• Key Item: Bishamon Ocarina (this is the key to unlock Bisahmon Sword)

Others can be obtained repeated clearing the Dark Realm.