I wasn't sure what you were discussing. Like, what is a fly tower? So I went down a barbican rabbit hole and found this part of a video, where one of the theatre fly techs describes the same thing as you!
The 'flys' are the enormous curtains and backdrops used in theatre productions. They are raised and lowered to set new scenes in a production. They need a large space to be raised into, above the stage--which is known as a fly tower.
https://youtu.be/uDdTUBk-qjo?t=140