People built walls around their cities for protection. Here Yahweh speaks of protecting Jerusalem as if he himself were a wall of fire around the city. Alternate translation: "I ... will protect the city, like a wall of fire around it" (See: [[rc://en/ta/man/translate/figs-metaphor]])
Yahweh speaks of himself by name to express the certainty of what he is declaring. See how you translated this in [Zechariah 1:4](../01/04.md). Alternate translation: "this is what Yahweh has declared" or "this is what I, Yahweh, have declared" (See: [[rc://en/ta/man/translate/figs-123person]])