This is the well known Loretto Chapel staircase in Mexico. Contrary to the claims there's nothing that defies Physics here. The staircase does have primary support in the form of an inner wood stringer of a small radius that it acts as a central pole.
There are several other staircases with similar design in the US & Germany too. Nails were not used widely when it was made, it's wood joining and wooden pegs that's used. Surely it's considered a very smart design but it doesn't break any laws of Physics.