No, no, wait! What if the tower was...
Just kidding. I'm one to admit when I'm wrong. Lead shot was made, but I've heard of lead cannon balls in times of desparation. Perhaps with it's low cooling point, they could have been made as such? In any case, our ovoid subject here was not a shot tower ball.
Ballast I suppose seems possible, but I would think that there were much better uses for iron, even pig iron. (early blacksmiths even fashioned tomahawk heads from it). After all, the majority of ballast in the past had been stones, right?
Could this ovoid mass of iron have been used for anything else? A reserve of iron for cannon balls during seiges? As a giant paperweight!
Just thinking aloud...