If you have cheats on(commands, such as /gamemode 1) you can type in /summon EnderDragon and boom. Without cheats... too bad.
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You can't. Go to " The End " to fight the Ender Dragon.
Yes
The End dimension can be left if the Ender Dragon is killed, and a portal is created, or if you die.
There is not an End Castle in minecraft, the End is basically only to fight the Ender Dragon. But maybe we will see a Ender Castle in further updates.
The ender dragon drops it when defeated