Minecraft wiki entry for Shelters, associated text: If a moat does not feel sufficient, build a lake around your base, preferably several levels below the entry level and with a bridge to allow easy access back and forth. Again, it can be water, or, if you want a dramatic flair, use lava. Remember to protect the bridge well, though, and be sure you are protected from within the island in case you miss a spawner. Making a drawbridge out of trapdoors is a fun alternative to the actual bridge.