Minecraft wiki entry for Multiplayer_PvP_bases, associated text: if made out of chests and trapped chests, it is (almost) completely undetectable without a hacked client. On the other hand, you might want to create a few defenses for your base.
People don't look upward as much as they look forward and around, unless they're adventuring and want to keep track of the time.
Raiders will have to use up lots of scaffolding to build a cannon outside your base.