You’ll find the Strongbox (also called the Iron Safe, depending on your version of the game) during chapter 3, in the cave to the north of Lakanaba. This item is not mandatory, but it will make the entire chapter much easier. 

Preparations

• Be at least level 3. After you get to buy your first weapon, kill random monsters outside of Lakanaba and you’ll get there in no time.
• Gear up. Whenever you kill a monster with Torneko during this chapter, they’ll leave a lot of loot. You can use it to gear yourself or sell it to buy proper equipment.

Finding the Cave

When you start this chapter, talk to the inn’s owner. He’ll tell you about a box being safeguarded in a cave to the north; that’s where you need to go. 

To reach the cave, exit Lakanaba and walk northeast, following the shoreline. Skip the first small peninsula you see and get into the second one. At the tip of the peninsula you’ll find the cave.

Getting the Strongbox

Walk west from the entrance to get to the next floor. In here, push the button on the eastern side to unlock the dam and get washed away to the next floor.

Follow the pathway to the next stairs and go to the northwestern corner of this floor to find a chest with a Chain Sickle. Equip it if you haven’t bought the Cautery Sword yet. 

Press the new button to slide down to the next floor, and get through the left path on the bifurcation. Walk west from here and you’ll trigger a rolling boulder trap. You’ll have to walk south and turn east to make the boulder continue and fall into the pit on the left side, making a bridge that you can cross.

Walk over the boulder and get to the next floor. You’ll find the chest with the Strongbox inside, but before grabbing it, you’ll have to push a boulder from the hallway into this room. When you grab the Strongbox, the door will close itself, but you can place the boulder in the same square to open the door again.

Congratulations! With this item in your bag, you won’t lose gold whenever you die from monster encounters. This is quite useful during this chapter, as most of the story will progress by spending gold on things. The last part of this chapter asks you to spend 60.000… imagine losing half of that in a fight!

Now that you have the Strongbox you can safely exit the cave and continue to do the next step in Torneko’s story: visiting Ballymoral to help the prince.

Note: you’ll lose this item after finishing this chapter. If you skip it, you can find it again in Chapter 5 but it will lose its use, as explained in-game by the wearing down of the box.