Great Gate of Minas Tirith

The Main Gate of Minas Tirith was the city's primary entrance point and its only weakness.

War of the Ring
During the Siege of Minas Tirith, the Mordor armies attempted to smash it down with a conventional battering ram, but the brave defenders beat them back. In response, the Witch King ordered the presumably bewitched Grond  to destroy the gates, which succeded, resulting in a battle in the courtyard of the First Level.

Aftermath
After the Ring was destroyed and Aragorn  crowned king of Gondor, the dwarves  rebuilt the gate using mithril , making it unbelievably strong and nigh impregnible.