Glassteel is crystal or glass that has been hardened by the 8th level arcane spell of the same name (see 1st edition Players Handbook, pg. 90). When applied to glass or crystal, it turns it into transparent substance which has the tensile strength and unbreakability of actual steel.
The glassteel spell was created solely for use in hardening the glass chambers used to store life energy in a Life Forge. When the spell became more readily available, it found more mundane uses like for hardening windows in the homes of the wealthy, wizard towers, on the ornate glass used in temples, palace chambers, and even in fortifications. When used in fortifications it can be valuable to defenders by allowing clearer view of what is happening in the area while affording protection from most attacks.