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In vanilla having stone walls AND playing battle means lots of dead enemys from arrows, balistas, oil, etc... Also it is almost imposible to ram gates.
If you have phalanx of coz you should defend streets near c. plaza, if good swordsmen, then walls (unless enemy had machine-gun towers:)).
On campaign you have 2-4 turns additional time to get reinforcements, aslo I always add law bonus to the walls in EDB.
You can choose wooden walls if you playing macedonia or seleucids with their very-long-spears (you can cheat if place them near gate and possible breach).

advantageous for ai - only when you storm cities with stone walls..