A fortress is a building designed to be defended, which means it was very hard for enemies to get inside. It kept the people inside safe. There is little difference between a fortress and a castle. Fortresses normally have very high, thick stone walls. There were very small windows, without glass, so that soldiers inside could fire out through them. As time passed, the weapons they used changed, from bow and arrow, crossbow, gun or cannon. Because these windows were so small, it was very hard for the soldiers outside to fire in. The people defending the fortress were also able to drop things like rock and boiling oil on the heads of those below.