I have always wondered why the Great Wall of China was as advantageous as it was to the Chinese. It spans so many thousands of miles and doesn't seem overly tall (around 25 feet high in many parts?). If an invading army or horde would just rush to a particularly remote section of the wall, they would be faced with (at first) only the guards of that particular section's towers, while the horde would number in the tens of thousands. Bring workers/slaves to create a ramp with the soil next to the wall, all the while fighting off the vastly outnumbered guards.
My estimation is that a working force of hundreds, or thousands of men digging to make a properly wide soil ramp to the wall would take on the order of days, while a proper Chinese fighting force to be alerted, assembled and brought to the particularly remote stretches of the wall would take on the order of weeks. I am just not understanding why a fast, "punch-through" tactic wouldn't have worked.
In addition to jianq's comment about the fires, keep in mind that knowing an attack was coming was sometimes easier than you might think. An army is quite hard to keep hidden and concealed. Scouts did a good job of locating anything out of the ordinary. An attack of that scale would also involve a huge investment of resources and men. This is something I'd bet China would have eventually taken notice of.
Further more, the Great Wall does more than just keeping the opposing army out. Even if the army makes it over the wall, any supplies or reinforcements have the inconvenience of traversing the wall. It also does not provide any tactical protection for the invaders once they have crossed over. Retreating back is made quite difficult by the wall as well.
There wasn't a much remote part, if a tower was attacked, they would light a fire and the next tower would see and light their fire and so on, and the enemy would be alerted to their movement