Hoarding/Stronghold 2

Round towers and square towers in Stronghold 2 can be built with an additional layer of hoarding that protects units on its top level from fire damage (predominantly from bolts coming from fire ballistae). Such towers cannot be placed a ballista or mangonel on top, and regular projectiles can still hit units standing around. When the hoarding is destroyed, the tower is converted to a normal variant.
Due to its very niche purpose, hoardings are usually considered infeasible to install.