Extending over 6,000 kilometers across China, the Great Wall of China was built to protect china from the barbaric enemies of the north. In that era the designers of the wall made it so high it could not be scaled, so long no one could go around it, and so thick you could not go through it.
But when the enemy breached the wall, it was nothing to do with the design or construction. The barbarians simply bribed the sentries or gatekeepers and the enemy simply walked in.
Bribery can catch us off guard, especially when we are tired and weak or when we least expect it. In Matthew 4, after 40 days in the wilderness, the devil tries different tactics to get Jesus to yield to temptation (including bribery). Through quoting the Bible Jesus resisted and stood firm.
More information from a historical view of the Great wall of China can be found here!
More information from a tourist view of the Great Wall of China can be found here!