A few games back I managed to destroy the Great Wall Wonder by accident when bombarding a city with artillery. I thought Wonders couldn't be destroyed - except by razing the city of course. Is the Great Wall an exception? As it happened this was a good thing, as all the Walls in the enemy <7 towns disappeared as well. But I already had Metallurgy, which makes the Great Wall obsolete - what happens to the improvements granted by expiring Wonders (Great Wall and Temple of Artemis) when the Wonder expires? I'd heard that, on expiry, all the improvements disappear - this is why I never build these Wonders myself, as I'd hate to rely on Temples to keep order and have them all suddenly disappear. But my game seems to indicate that the Walls don't disappear when Metallurgy is discovered. Maybe the expiry of the Wonder means that new towns/newly-conquered towns no longer get the free improvement?