This is for the people who are having troubles playing siege battles in custom battles (with pictures!):
FOLLOW THESE INSTRUCTIONS EXACTLY!
1. Select a map without the little silver box next to it.
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2. Set the settlement level to "Fort"
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Note: It will say "none" for settlement level after this. That's alright!
3. Select your culture (such as Empire)
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4. Re-select your settlement level (such as Huge-City)
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5. Play the siege battle!
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Have an enjoyable siege!