Minecraft wiki entry for The_End, associated text: Once the player enters the End, the only way back is to die or defeat the ender dragon. The dragon spawns naturally and flies around above ten towers of obsidian arranged in a circle around the central Exit Portal, which fills up at the end of the battle. On top of each tower lies an end crystal, some of which are protected by cages of iron bars. These crystals heal the dragon, but can be destroyed by hitting it (even with projectiles). Destroying the crystal while the dragon is healing damages the dragon.