You may find that you will be so dehydrated while climbing the Great Wall during summer and will rush to buy the bottled water from the local traders around the wall.
Be aware that they tend to sell you "fake" mineral water which is actually tapped water in empty bottle collected from the bin. Most of them will freeze the bottle into ice and sell it to you as if it is a mineral water from the water plant.
Take a close look at the bottle cap and make sure that it has the standard leak-preventive ring below the cap. Otherwise, that is a re-use bottle.
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At Least Do This: Carry one bottle water with you before you leave the city.
Alternative: Otherwise, don't buy mineral water but get other types of drink which are less likely to be fake, for example Green Tea, Ice Tea, Coke, etc etc.