What's the meaning of 帖子的顶楼？ I know 帖子 means post or thread on internet forum.

What is 顶楼? Is it the 1st reply?
As I understand it: 

一楼 = first post

顶楼 = latest post

We often see post refer the previous posts as 楼下

Take this page for example:

85樓 (2019-10-07 02:23) is the latest post

2樓  (2018-05-29 15:49) is the oldest post (一楼 must had been deleted)