There are what appear to be polished basalt / granite inserts into the sides of the Basket Towers, which are otherwise made (as are the walls and stairs) of irregular sandstone blocks precisely fitted without mortar. I don't think they can be merely decorative, and suspect that when the towers were under construction that they played some defensive role, perhaps to discourage overflight. They are so different in composition and finish from the rest of the structure that one might guess they were the remnants of another more ancient style of architecture entirely.