A fortress is a building designed to be defended, which means it was very hard for enemies to get inside. Fortification kept the people inside safe. There is little difference between a fortress and a castle. Fortresses normally have thick stone walls, thicker but less tall than in castles so artillery cannot break them. 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. Soldiers outside had great difficulty firing into such small windows. The people defending the fortress were also able to drop things like rocks and boiling oil on the heads of those below. Successful attacks were usually sieges.
Fortresses were not designed for comfort. They were dark and often damp. The Tower of London is an example of a fortress.