We went to the Museo do Pobo Galego and happened to walk into this staircase.
We didn't notice how weird it was until we got lost on it. Later we saw it on a post card.
The building itself goes back to the thirteenth century but the staircase is the work of Domingo de Andrade, and it dates between 1685 and 1715.
The museum website says:
"Inside the building, at the northwest corner, is the triple helical staircase which, for its technical and structural audacity,
is a good example of the virtuosity of Andrade and well worthy of our admiration."