1. Broken Stones: Careful, you may fall for this if you have a weak eyes.
2.Stone Surrounding: If they fell, they will most likely to stuck there. But if they had a bomb, then it would be a different story.
Resources:
+Stone: 374 (can be reduce if not use the stone surrounding.
+Wood: 120 (can be reduce if build 1 bridge, skip one, and build another one...).
Of course you will need to make it a little harder to get over in case some knight use slash-jump