The End dimension can be left if the Ender Dragon is killed, and a portal is created, or if you die.
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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.
There are 2 portals you can craft in Minecraft. The Nether Portal and The End Portal.
You cant yet Minecraft Xbox 360 edition hasn't received the update where you can go into the end but I believe it will be coming soon.
On a Minecraft server, chat is always on. If one of your friends can't chat, then it is a problem on their end.
No