‘Her light was like a most precious stone, as it were a jasper stone, clear as crystal, having a wall great and high, having twelve entrances, and at the entrances twelve angels, and names written on them (the entrances) which are those of the twelve tribes of the children if Israel.'
The light of the city was bright and eye dazzling like the light from a diamond or opal (in John's time ‘iaspis' could represent a variety of stones and we must decide from the context which one is intended). Compare Revelation 4:3. ‘A wall great and high'. This signifies her total security, she is under God's protection.
‘Having twelve entrances, and at the entrances twelve angels, and names written on them which are those of the twelve tribes of Israel. On the east were three entrances, on the north three entrances, on the south three entrances and on the west three entrances.' The twelve tribes of Israel were described in Revelation 7:4-8 where they represented the true people of God, the new Israel, the church (see on those verses). Thus the fact that their names are on the entrances demonstrates that this is their city and their abode. Enter in and we find the people of God. The twelve angels at the gates, like the high wall, demonstrate that they enjoy God's full protection.
Each side has three entrances on which are names of tribes of Israel. Three is the number of completeness. Compare Numbers 2 where the tribes of Israel were three to each side of the Tabernacle. The same idea is found here.