Dark Souls Remastered | Archive Tower Giant Cell Key location & where to use (Duke's Archives)

The Archive Tower Giant Cell Key, not to be confused with the Archive Tower Giant Door Key. The latter (the Door Key) is necessary to progress through the game. By the time you get to the bonfire on the balcony near Crystal Lake (also known as Duke's Archives 3), you've already used it.

Here's where to get the Cell Key from Duke's Archives 3, then how to backtrack from that bonfire to the spiral staircase and where to use it. The loot you get from opening the cell is a Fire Keeper Soul, which is not essential to progress through the game, but it's very useful.