Minecraft wiki entry for Building_a_metropolis, associated text: Underground: Build your entire city underground. One way to do this is by building a network of tunnels where the people live. The city can become quite maze-like if done this way. You can also build the city in a cave. An interesting twist is to have the buildings hang from the roof of the cave. One way to do the lighting system is to connect daylight sensors to redstone lamps, or if the cave is close to surface, building a giant skylight out of transparent blocks such as glass . It is also possible to build this city inside a mountain, and if you choose to do so, it is possible to have windows on some of the walls as well. This city could also have a few mines in it. A city built underground should be quite easy to defend and even easier to hide, unless you chose to build the giant skyglass, as only the entrance will be visible to the surface. If you want the city even more fortified, don't build any entrances leading to the surface and make the only entrance through a nether portal. Lava fits well to an underground city, and can be used for both defense and a light source.