What does Revelation 21:12 mean?
The New Jerusalem is secure. Nothing and no one except what is righteous is allowed to enter (Revelation 21:27). A massive high wall with twelve gates surrounds the city. Twelve angels guard the gates. The gates serve not only to symbolize security, but they also serve as memorials. The names of the twelve tribes of Israel are inscribed on the gates.Walls and gates were used in Bible times to fortify cities. Wooden or iron gates closed at night to keep out enemy soldiers and/or plunderers. Inside the cities were narrow streets and courtyards. Buildings stood around the courtyards. Of course, enemies were known to scale the walls and crash the gates. When Jerusalem fell to the Babylonians, the city's walls were left in ruins. It became Nehemiah's mission to return from captivity to lead a delegation in the rebuilding of Jerusalem's walls (Nehemiah 1). The wall of the New Jerusalem will never fall, and its gates will never be breached.
This, of course, is meant as a highly symbolic gesture. In eternity, there won't be any wickedness or unrighteousness to be protected from. Nor will there be any evil, danger, or harm from which to be protected. The existence of these walls, and gates, serve as real-but-symbolic emblems proclaiming the safety and security of those living in New Jerusalem.