John must note that at each gate stands a single angel. This contrasts with the plurality of angels (with their flaming swords) who guarded the way back to Paradise after humanity’s exile after our fall into sin (Genesis 3:24). The singularity of an angel at each gate speaks to the ultimate absence of danger (recall the demise of all enemies, including unbelievers, Revelation 20:15) and in fact the present defeat of the dragon and his allies (Revelation 12:7–9). More, it speaks to the perfection of the protection the Lord God himself would give to this city. So these twelve angels can also be seen as a welcoming committee for those who would enter (Luke 15:7, Luke 15:10).
12 It had a great, high wall, with twelve gates, and at the gates twelve angels, and on the gates the names of the twelve tribes of the sons of Israel were inscribed