Consult these maps for part 1:

You started the game at an entrance to the Keep (Nanamagari). Just follow north as the soldier instructs you to do, then you will face two Three-Eyes ninjas. Just hit them as hard as you can, but you cannot kill them though. After they fled, watch a cutscene where you reunite with the princess Yuki and automatically receive a gauntlet. Unfortunately, she is kidnapped by an Ogre King, Osric.

• Key Item: Gauntlet
• File: Instructions

There is a Magic Mirror right next to the door where you can use it to save the game and enhance your weapons/items. 
Enter the door and continue the path until you encounter a group of Zombie Soldiers, kill them and enter the room. Inside, you will have to fight another group by Zombie Soldiers. After you are done, Exit the room to trigger a cutscene, one of the soldier will direct to the forest which is a shortcut to the Keep. Enter the forest, kill the Three-Eyes who jumps at you at the cave's entrance. Before you enter, pick up Herb from the ground in front of the cave.

• Item: Herb

Inside, you will find a hole, examine it but don't jump into it yet if you haven't completed "Undying Summer" and "Fall's Flight" achievements. Skip this part if you already obtained the achievements mentioned above.

• Achievement: Fall's Flight
  Walk all the way back to the bridge where you first encountered a group of Zombie Soldiers. Stand in middle of the bridge for about 30 seconds to 1 minute, you will see geese flying and an achievement notification will pop up.

• Achievement: Undying Summer
  Going back to the forest. On the second screen where you see large tree on both sides, stand near the 2 large trees on your right and press O to absorb souls for about 30 seconds to 1 minute, you will see a beetle crawling up the tree and the achievement will be unlocked.

Now enter the cave and jump into the hole. Advance forward, kill Three-Eyes, then pick up a map from the treasure box near the staircase.

• Map: Underground Temple Map

Go upstairs and examine the mummified statue to open the door behind, also you will receive Sougen's Note automatically.

• File: Sougen's Note

Enter the door you just opened. From Samanosuke's perspective, turn right to the corner and kill a Zombie Soldier. In this corner, you will notice 2 small opened doors on the wall to the north and east. The one to the north contains a Fluorite.

• Item: Fluorite

Now turn left you will reach a dead-end with another small door on the wall, it is closed this time so open it to get Seiryu.

• File: Seiryu

Go to the opposite side of this hallway, you will find a door, enter it. Inside this room, there are 6 blue jars, destroy the one at the top-left to reveal a treasure box. Next, pick up the Thunder Orb (Shiden) in the middle.

• File: Journal #1
• Equipment: Raizan (Shiden)

Continue the hallway. to your left from where you picked up Thunder Orb, you'll notice a large trick box. 

• Item: Rope Ladder (Solution: left, right, left)

After you get Rope Ladder, continue upstairs. Immediately after you get out of the underground, you should notice a red box in front of Samanosuke. It contains Magic Jewel and you should use it immediately unless you are doing no upgrade achievement.

• Item: Magic Jewel

Now follow the path you'll find a puzzle box and Magic Mirror on the left side. The box contains Power Jewel (use it immediately to increase your health bar), then save the game.

• Item: Power jewel (Solution: 1st option, 3rd option, 6th option)