Z – Z Axis
As Kirk and Spock discussed in Wrath of Khan, don’t have two dimensional thinking.
Think of below the street level and above the tops of the towers.
Is there an issue with burrowing creatures or flying creatures?
Would a dragon or someone riding a dragon attack the city?
Is the ruined city so isolated by geography that the only way to arrive is by magic or by flight?
Is it a living but hidden city?
Is there a magical effect above the city, like the area of a rope trick, or other pocket dimension type space that is hidden from view? A place for observation of the surroundings and a great spot to ambush invaders, or trouble makers.