Where is the cell "old cell key" unlocks?

Question

I found "old cell key", but I can't find that "old cell". Where is it?

This is crazy but it seems like everybody in the internet shows how to find the key (which is super easy with out any hints), but noone care to show how to find the cell.

Answer 1

Found it.

You must go down through a hole from the bonfire at the Profaned Capital, drop down to the toxic lake, climb to the roof and then instead of taking long stairs up, you need to find a window next to the roof and jump into it.