How would Roman Warfare have changed if they actually had legendary 'flexible glass', and it was cost-effective?
It's an unbreakable, and I mean literally unbreakable, glass. Assume that it looks and acts like normal Roman glass, so an enemy could melt a hole through it, if they wanted, and can be made at about the same price, but it just can't shatter, and instead dents like brass. If you can make glass, you can make vitrum flexile, with just the addition of some additive, which Rome won't run out of.
Could you make city walls out of the stuff?