Why would this city have a great, high wall?

Revelation 21:12 (ESV)

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

The second thing John sees about this descending city is its wall, or better, the sheer size of the wall. A city in John’s day characteristically had a wall surrounding it, to serve the dual purpose of designating the boundary of the city as well as providing protection from enemies. A wall need not be great and high to designate the city’s perimeter. So the addition of the two adjectives points in the direction of protection, and the degree of safety inhabitants may feel. We might be inclined to respond to this understanding by referencing the fact that John has already seen all enemies cleared from the earth (Revelation 19:20; Revelation 20:10, Revelation 20:14–15) and so a protective wall is utterly redundant. Yet it is precisely this observation that underlines the comforting significance of this wall. Consider:

With the wall God symbolically reassures his people (the inhabitants of the city) that their relation to him is unbreakable, indeed, cannot even be challenged; he is her protector. The psalmist lingered around the walls of Zion, seeing those walls as reassurances of God’s endless faithfulness to his covenant people (Psalm 48:12–14). Furthermore, Isaiah saw the city’s walls as a symbol of God’s salvation (Isaiah 26:1) and even said that the walls would be named Salvation (Isaiah 60:18). In the present vision, then, we may understand this great, high wall as designating the safety the Bride may experience because of the Lord’s own personal and eternal protection.
This great high wall is one of several pointers that the vision of the New Jerusalem (Revelation 21:9 – 22:5) must not be understood as descriptive only of the time after the Last Day. What the Lord is describing here will certainly receive its fullness after Christ’s return but outlines of what the Lord will bring to pass at that time are evident already in this dispensation. The high walls of the church denote the safety the Lord provides his people inside his church. This is a very safe place, and so needs to be a very safe place.