This gate will open during the day, and shut at night. At the top, there is an override switch to shut it during the day. With 1.8, you can now just right click the sensor if you want it to open at night and close at day. If you would like to MCedit this into your castle, fort, etc. Please pm me first.

Additional Notes

Thanks to minecraft.gampedia.net for the idea. (I did all of the building myself)