The fortified tower at the heart of the castle is called the keep. Many keeps were used as residences for the lords and nobles who owned the castles. These keeps were symbols of military and political might. While also serving as a refuge of last resort should the castle be breached by invaders.
Many of these keeps were thought to be impenetrable and were so for many years. Until the invention of gunpowder and more modern artillery which ultimately rendered these towers obsolete. What were once impenetrable fortresses out of range of any siege weapon or projectile could now be destroyed from a distance.