Glass is a kingdom resource that you can get when you reach level 50 in the game, and open your castle in the Fire Peaks kingdom. You can produce glass in your castle by building Glass Furnaces, which is boosted by Glass works.

Also, you get the glass as plunder when you raid Cultists Towers (equivalent to Robber Baron Towers in Green Kingdom) in Fire Peaks.

The glass you can use in the village in Fire Peaks - and once you reach the Legendary levels (level 70+) - the glass becomes an important ingredient in building new, improved buildings.