*spoilers* I have the darm key...what next?

[NullBy7e]

EDIT: nvm missed boss in the mines.

Do I need to make way towards the Darm Tower?
the last boss I killed was in the Shrine, that worm thing and I spoke with every NPC I could find it all towns.

Also gave Reah the Harmonica, I spoke with the two trees and I freed Feena with the Prison Key.

I can't figure out how to open that door at the Thieves Den.

[Mondae Sundae]

The Darm Key doesn't open the door to Darm Tower. If my memory is correct, it opens the door leading to the boss in the mines.

You need to fight the boss in the mines before you can go to Darm Tower. You also need to 'read' all the books of Ys before you can go to Darm Tower. The door in the Thieves Den won't open until these conditions (and a few others minor ones) are met.