Minecraft wiki entry for Nether_hub, associated text: After your map is finalized, go ahead and move your portals to the desired locations. You will have to dig up into the ceiling to get the portal to fit. To prevent mob spawning, put glass all over your map. You can still see your map, but mobs will not spawn on glass. The portals should be level with the glass so you can just walk right in. If there is any area where mobs are likely to walk into your map, block it off and use an iron door if you will need to enter/exit. Light up the area around the overworld portals too so mobs do not enter through the portals.