We might be talking about two different buildings. I’m talking about the one with the big “mushroom” top — square, but with a sort-of mushroom profile, with a couple of outsized floors at the top, as if someone wanted extra penthouses up there.
There was an odd office tower over near the post office and rail station that had oversized top floors. Loyola Avenue, I think... don't know if that one is gone (it probably should be).
The ITM building (the first of the World Trade Center buildings) is an x-shaped tower with a smaller, round structure on top. The round part used to slowly rotate, and had a very "Mad Men" bar/nightclub that was called "The Top of the Mart". The most recent club in that space was "Club 360" - its website is still active but I believe the club is closed. IIRC, there was a separate elevator to the "top".
The city had proposed that the building be used to house "disadvantaged businesses", which killed any outside investor interest. Attaboy, Mitch.