The NAND Gate is a redstone circuit, that only opens and closes when you make all the switches go up/down. To close it you have to have all levers down, and to open it you have to have all levers up.This type of NAND Gate has 4 levers, two in the front, and two in the back. This is just an example of a NAND Gate with pistons that lead into a chest room, the structure itself is 11x12 and is very stylish ;) .

Don't forget to 264 [look up data values on minecraft wiki ;]