Minecraft wiki entry for Shelters, associated text: It is possible to make a drawbridge using sticky pistons that is very difficult for a mob to cross. This draw bridge also involves the use of a moat or lake. A knowledge of redstone logic gates and how power is supplied to blocks is needed as the mechanism uses multiple redstone repeaters, NOT Gates and a T-Flip Flop. Using the T-Flip Flop one can wire the drawbridge so that it becomes accessible or inaccessible with the touch of a button from either side of the bridge. When the player(s) wants to cross the button is pressed causing both ends of the drawbridge and the center to be even allowing the player(s) to cross. When activated to keep mobs out both ends of the drawbridge are raised 1 block higher than the water level making it impossible to jump from the water to dry land and the center blocks are lowered 1 below the water level causing the center area to flood. Assuming both areas are properly lit and secured this creates an impassable entrance to one's base.