It sure seems like a completed structure at first glance. But look closer. Specifically, look at the piles of stone blocks stacked on top of the columns.
Those blocks were hauled up there during construction—around the turn of the 20th century. They were supposed to be carved into sculptures representing "Music", "Architecture", "Painting" and, ironically, "Sculpture". Instead, the stone has sat there for 110 years, through two major renovations, un-carved and largely ignored.