Minecraft wiki entry for Defense, associated text: There is no particularly expensive way to defend against ghasts. Glass or cobblestone (which ghasts cannot damage) will do just fine, but you could also use an expensive material like iron blocks, or obsidian. However, while glass panes protect you from being noticed and fired on by ghasts, they are shattered in the event a stray fireball explodes near the panes. Glass blocks, on the other hand, do not shatter even when hit directly. To fully ghast-proof a Nether fort, use only glass blocks, and use iron bars or nether brick fences for windows.