I bought and moved into a house six weeks ago. There was some semi-built-in furniture which came with the house, but it was gross, so I had it removed recently... Which leads me to this. There's an iron door which opens to the interior of the cinder block wall. No obvious purpose, no visible pipes or valves or anything. What might this be?
Fireplace ash cleanout.
In the back center of the fireplace, there will be an ash drop opening.
The popularity of these date to when the fireplace was used 24 hours a day. You could safely drop ashes with hot coals, and later remove the cold ash.