When you get to the part of the small drawbridge and you start lowering it, walk on it anyways. The bridge will stop lowering. It does so every time you step on it. Wait until the bridge is slanted perfectly with the roof of the building overhead so that it acts as a ramp. Once so, you can now cross and can get on top of this roof. The zombies approach from only the corner of the building, allowing you to pick each one off. This is a better way to conserve health at this point.