I work in a glass factory. As far as "unbreakable" glass...no, not LITERALLY, but we can do laminates using security wire and various interlays that are massively inconvenient to get through. Just as an example, we did the glass in the polar bear exhibit at the zoo...it's designed to take the weight of all that water PLUS the impact of a SERIOUSLY pissed off half-ton bear...and then some.
The problem is, when you get to glass that strong, it's going to be THICK... then mounting and securing it becomes the weakness.
Honestly, steel is the way to go...ever see a bank vault with a glass door?