Once you have completed the game, a new mode is unlocked that lets you fight other human players. Clicking a crystal in your inventory transports you to a new 'world' with a single base inside a compound. You can transport out again by clicking a crystal again, and you are taken back to where you left the original game. There are no hostile animals but there are a very few zombie soldiers.
What makes it different is that other real players will appear, & then the fun starts!
This mode can be a little frustrating at times, since the sync between players isn't always very good. Sometimes you attack a player who seems oblivious to your presence, and your hits don't register, or you see him blasting away in what (to you) is obviously the wrong direction. Alternatively, other players sometimes seem to be able to kill you remarkably easily, & this seems to be either a sync problem, or maybe some 'cheat mode'. But when things are working properly, MP mode can be fun.
Multi-Player on iOS Edit
To my knowledge, since the last several updates on the iOS version, multiplayer is not enabled, and will not work when a crystal is used. Please correct me if I'm wrong on this. -4Doug it works now . From the Gamer4lo
This mode still works on iOS.. played it today (Aug 2016) For some that may want to test the limits of the multi-player island, let me save you some time.. the huts on the island do not become fast travel stations, and the further away from the main tower you get the slower it seems like you walk.. also, if you walk across a bridge, it will take forever to get to the other side and when you are almost there, it will dump you in the water back at the side you started on.