Instructions
Find an area for your drawbridge. The area for the drawbridge needs to be completely flat and four blocks wide.
Place a line of blocks on the edges of the area so that the middle is two blocks wide.
Dig a trench in the middle area.
Place trapdoors on the top rows of blocks. Trapdoors are made by placing wooden planks in the bottom two rows of your crafting table.
Put a line of redstone dust on the tops of the blocks the trapdoors are attached to. Connect the two lines at the end of the drawbridge.
Place a lever where the two lines of redstone dust connect. When pulled, the lever will cause all the trapdoors to open, preventing other players from passing. A lever is crafted by placing cobblestone and a stick in the crafting window.