When constructing the bridge, you need to use the WASD keys to define which direction (if any) it raises. You then need to build a lever (Anywhere, it doesn't matter where, I tend to have a dedicated room for my levers, color- and proximity-coded for purpose) and link the bridge to the lever using the lever's q menu and two more mechanisms.Raising versus retracting is mostly a matter of taste, but keep in mind a raised drawbridge will act like a wall.
Edit: You'll get this message if you try and violate it, but a raising bridge needs to be supported along the full length it will raise on.
[ May 25, 2008: Message edited by: Dareon Clearwater ]