why is there no corner in a circular room

Corner is a point where two straight edges of a figure meet, and a circle has no straight lines / edges, so it can have no corners / vertex. A circle can be seen as the limit of a set of polygons with increasing numbers of edges. If we approximate the circle by, say, a "polylgon" with a huge number of edges,(where number of egde is tending ti infinity) then we will have many microscopic corners. But when you go from polygons to the actual circle, you no longer have any edges or corners, only a smooth curve. Without edges, you have no angles or corners.

Hope this would have cleared your doubt .