What do I do about these invaders on the Great Wall?

Hint 1 of 4:

Maybe you should warn someone.

Hint 2 of 4:

You can't find any guards, so make them find you.

Hint 3 of 4:

Do you have anything that could be used as an alarm?

Hint 4 of 4:

There is something in Dover, 1361 BC.