Sandcastle, sand dreams

Too simple outside, too complex inside.
My sandcastle is bigger than a real castle
But it only measures twelve inches
I was there once
But the sand made me forget how it was look like.
I built it in five minutes,
But I still can't destroy it.
I used all the beach's sand to bouild it,
But seems no one can see it.
My sandcastle has one hundred rooms
And one hundred windows.

My sandcastle appears when I'm feeling alone
And sad, when I need to go running
And I don't know where to go.
My sandcastle doesn't exist in my mind,
It's real, If you don't believe me,
Close your eyes.