If you are looking at “The School of Athens” by Raphael, that appears in one of several rooms with frescoed walls by Raphael that were private chambers for the popes. People are often confused looking at reproductions about what is really architecturally and what is painted (trompe l’eoil). This corder of the room has a pendentive structure in the corner with a kind of capital sticking out from the wall that you can see almost at the top of the photo. The rest is painted- figures as well as architectural elements. At the bottom left corner of the photo is a doorway with marble framing but the rest of the “marble” is painted.