If you are building a castle in minecraft pocket edition then you need aplace with flat grould and no mountains. When you are building your house you need a flat surface too but here's a secret. when building you house you can trick your friends. Make a tiny little square building but inside put nothing but a trap door and dig a huge, underground house so make sure the walls are like stone or something ( has to match what you used to build the tiny building of course! and when you show your friends the outside it's small but on th inside it is huge!! NOTE: MAKE SURE YOUR IN CREATIVE MODE!