What's the 1 inch square knob sticking out of the wall?
It looks like a sewer cleanout plug:



Scrape off the paint around the circle and you should be able to turn it with a wrench. Though sometimes it can be a bit hard to turn, particularly if it has been painted over and untouched for many years...

However, you might never need to remove it. You only need it if it provides a good location for cleaning a blockage in the pipes. But you definitely want to know where it is for when you really need it.