Okay, this will be a short post. But I have this video by a certain Rob Whee that shows that crosses, being the internal frames of tropaea (tropaion's / tropaeum's, i.e., trophies), were sacred to the Romans. Sometimes you'll see even plain crosses on ancient Roman coins that have nothing to do with Christianity.
Here is what Rob Whee has to say about crucifixion in the video:
The tropaion was a sacred symbol of victory. The trophy was a tribute to Victoria. She was a winged lady deity that flew over the battlefield and granted victory. Winged deities go as far back as ancient Sumer. The empty armor on a stick would be filled with the spirit (air) of victory. The ancient Greeks would set up the trophy on the battlefield, and left it standing. The ancient Romans would set it up for their triumph parades in Rome. Enemy armor, and prisoners would be placed at the base of theis sacred symbol that represented the Sun's life giving rays.
Research crucifixion beyond the history books, Wiki pages, and scant physical evidence. I find the Latin / Greek texts do not line up with English translations from what I can see, The nail in a shin bone that apologists will try, and make look like there is many examples of, falls short. There is only one example that I know of, and the nail with the bent over tip is way to short to attach to anything solid enough for such a task as crucifixion.