At the top of/on the top

[Xinfu]

I know that
at=inside or outside or around; in=inside; on=touching the surface,
but
-He wandered round the base of the enormous fluted column, at the top of which Big Brother's statue gazed southward toward the skies [...] (115)

Here the writer is talking about something touching the surface of the top of the column, then why at not on?

[poli]

He could have used on, but at is good here too. At is often used in describing a location.

Examples: I'm at 46th Street and 8th Avenue. I'm at the top of Mt. Washington. On can be used here too. There are many rules regarding
prepositions, and at and on are not always interchangeable. For instance the cherry is on the cake not at the cake.

[aniRB]

"At the top of" implies a specific location. In contrast, "on" can mean "anywhere on" but not necessarily "at the top of."

For the example of a mountain, "On the mountain" could be near the bottom, half-way up, near the top, at the top, or anywhere in between. However, "at the top of the mountain" gives a specific location at the top.

So, for your sentence, which is about a column, which is narrow, and it is unlikely something could be "on" it without simultaneously being "at the top of it" either "on" or "at the top of" are correct and they are interchangeable. So, then it just comes down to the sound of the sentence, possibly within the context of the larger paragraph, why an author may choose one and not the other.

-ani